C64 power supplies are way too expensive and if you have a couple particular skills you can build your own.

This assumes you have some experience working with electricity and have dealt with 110V wiring before, otherwise have someone else do it. I have the chops to do things half-ass; you'll know if you do too. If you aren't already confident you can check the voltage and polarity of a power supply and solder up a plug given the below diagram this is over your head for now, please do not attempt it, you'll hurt yourself or blow up your C64. I trust you to double-check which wire's plus before hooking anything up.

Above is the pinout of the Commodore 64 power supply. There are a bunch of types afaik but they're all the same on the output. I think the C or the 128 or both take different supplies but that's not what we're here about.

There's no special relationship between the voltages here. You don't need a "5VDC and 9VAC power supply," you just need two different power supplies, which you will put inside one trenchcoat. You can probably build this with just that information, but I'll write up the rest of this to fill in any unanswered questions you might have at this point and suggest some ways to source parts on the cheap.

The first component is the 5V supply. This is trivial to find: Get an Android USB charger with a built in cable and cut the connector off. There'll be a power and a ground, bob's your uncle.

The 9VAC supply is tougher. This isn't really a "power supply", it's just a plain transformer. Transformers output AC, so if you get a wall wart that's rated at 9V and sufficient amperage, that'll work. If it's DC output but definitely a transformer-type adapter you can convert it - I made a video tutorial on how to do this here and will convert the steps to a blog post soon. 

It's important to note that you can't go too far over the desired voltage with a plain transformer. If you connect a 9V@3A transformer to a C64, you'll be feeding it something like 14-16 volts. Bad idea. But if it's rated at about 1000-1500ma you should be fine.

One problem with using an AC adaptor for parts is if you take the transformer out and put it in your own chassis you'll have to attach to the prongs that would have gone into the wall outlet. It's not practical to remove them and attach to the transformer wires directly. If you want to be safer but spend more money, buy a plain transformer:

The advantage is that these have insulated wires on input and output that can be stripped and soldered like normal.

Only thing you need to know about this is that most plain transformers are center tapped, which means that on the output there are three wires instead of just two, and you need to use the two identical color wires. So in this case there's two black wires which you'll wire to the 110V input, and the output has two red wires and one yellow. Use the two red wires, put electrical tape / a wire nut on the yellow. Color code will differ by transformer and it's critical you identify which side is the 110V.

The hardest part to scrounge is the plug. You can get one on Mouser, and it's cheap fortunately.

The cable is easier to scrounge but still somewhat of a problem because 1-1.5 amps is a bit of power. If you don't want to try to source a cable and worry about whether it's safe, see the "Safer Design" at the bottom which presents fewer such questions.

For mine I used a 4 conductor cable with 20ga conductors that I bought at my local electronics store, a luxury many towns don't have. You might be able to scrounge something like this from some other power cable. You only need four conductors because there are three redundant grounds and one unused pin. I recommend this approach.

CAT5/6 might work in a pinch but the wire gauge is probably too small for the max power rating of the C64. If you do this I would bind the pairs together to increase current capacity, and don't leave it running unattended.

If you really don't want to solder up a DIN connector (can't blame you) you can maybe get a prebuilt cable. Here's one from Amazon, unfortunately; if you google you can probably find other sources. I don't know what the wire gauge is but I'm sure it's thicker than CAT5/6.

The way I built mine works as a proof of concept but is not safe. You can do this better if you use a transformer with its own wires instead of a reused AC adapter. There's another, safer way to do this detailed in the next section, and I recommend you read and use that approach unless you have a compelling reason to do it this way.

I started with a project box. You can use any plastic box for this as long as it's not easily breakable. I cut a hole in one end and installed an IEC power jack (you can scavenge this from a dead PC power supply) but you could also just drill / cut a hole, snip the end off a PC power cable, feed it through and tie a knot in it to keep it from falling out. Cut off the green ground wire (RIP) and strip the other two. Polarity is not important.

The PCB with all the parts is a 5VDC power supply I got by breaking open a 3A AC adaptor and pulling out the circuit board. There are two thick wires for 110V in and two thin wires for 5V out. There was a + on the board that told me which was which but if there isn't you'll have to put everything together, power it up, and use a voltmeter to determine polarity.

The two (thicker) input wires from the supply board are soldered to the hot and neutral lines of the IEC plug.

In the upper left corner is the transformer from a 9VAC wall wart. I broke the plastic casing, took out the transformer, and stuck two wires onto it with spade lugs, which slide right on to the 110V prongs:

   Spade lug  

I crimped these to some 18ga speaker cord I had and soldered the wires to the hot and neutral on the IEC plug.

Obviously you can't do this if things can move around and short out, so glue down both power supplies (I used hot glue) and insulate the joints with heat shrink if you can. You can't take the prongs off and solder straight to the transformer windings, it just doesn't work, and if you try to solder to the prongs the windings will fall off.

If you used a plain transformer, just solder the input leads to the hot and neutral 110V input.

Take the output wires from the 9V transformer and splice them to two of the conductors in the cable going to the DIN plug (polarity is unimportant), then splice the output from the 5V supply to the other two wires. At the other end connect to the DIN pins according to the diagram. Make sure you reverse everything left to right (only the 5V pin matters, everything else is symmetric). Split your one ground wire to all three ground pins with whatever spare wire you have laying around. You're done.

Since this was created using two separate power supplies, another way to do this is to just use two separate power supplies basically unaltered.

Get a 9V AC wall-wart adapter and a 5VDC adapter, don't disassemble them, just cut the connectors off. Get a 7 pin DIN connector from the Mouser link above and just run both cables into the shell. Solder the wires to the pins, and you're done.

This is exactly the same design as what I showed you above, in every conceivable way. It just uses two wall outlets. There's no reason not to do it this way if you have two outlets free, and it prevents you from ever having to touch 110V. If you aren't a big electronics person I recommend this approach, it eliminates all questions of amperage rating in wiring and insulation quality. It also makes it easier to replace a supply if one fails.