If you're looking for belts for smaller devices like personal stereos or memo recorders you could do worse than to buy an assorted handful of belts, also found on eBay and the like. These sets will contain various sizes, shapes and thicknesses, and there'll usually be one or two in there that fit your device. These sets are very cheap and handy to have lying around.
Putting the belts in can be fiddly, sometimes to the point of having to dissemble half the machine. There's no way around it, though. Try to find service manuals or tutorials on how to do it for your device. HiFi Engine is a good place to start looking (you can see if they have yours, but you'll need to register a free account if you want to download any manuals).
I'm aware that the picture isn't great, but I cannot be bothered to open up the deck for a picture as it's rather a hassle. I'll get to it, I promise.
Get out your contact spray (the kind that leaves no residue) and, whilst depressing the play and record buttons now and then, get that good stuff all up in there. If you're in luck, this will restore contact and your deck will play properly again.
If not, here's a simple test to see whether the issue is somewhere in the circuitry or a matter of the tape head: find the wires that come straight from the head and desolder them from the board where they attach. Switch them (the left wire soldered onto the right terminal, the right wire onto the left.) If the sound issue remains the same, there's a problem with the circuitry. If the imbalance is now switched left-to-right, it's the tape head. Don't forget to switch the wires back!
You may or may not be able to fix this. In older and more high quality devices, which were built to be serviced, you'll usually be able to take off the cassette bay door to access a screw that adjusts the positioning of the head:
The tape head is pointing up in this picture. The red arrow indicates the screw that is turned to adjust the head alignment. The yellow arrow indicates the record head, which can also be adjusted if necessary with the two screws on either side.
The process is simple; sit in front of the device with your headphones on, play a (properly balanced) tape and adjust the head until the audio is just right. Top tip: afterward, take a picture of the screw. The balance has shifted before, and it may again slowly slide, so this way you'll be able to see if it's caused by the screw turning. I've not had any such trouble, however.
If there is no way to adjust the head position, there isn't much you can do aside from actually modifying the device (by bending things, for instance) or the positioning of the cassette in the bay. You can screw around with spacers and bits of cardboard, but really, perhaps your machine is just a write-off at that point.
Fixing a cracked, gooey, sticky or otherwise broken pinch roller is no mean feat. First things first: if your pinch roller is still in working order, measure it. Find out exactly how thick and long it is so that if it should ever crap out, you'll know what to look for or work towards.
A sticky pinch roller is often an omen of what is to come, but the stickiness can usually be wiped off with a bit of cloth or some printer paper. This will technically make the roller a bit skinnier, slowing down your tape, but when we're talking fractions of millimetres, I think you'll be okay.
A gooey or cracked pinch roller needs to be replaced. You could conceivably build one out of electrical tape or restore the surface by sanding it down and adding a layer of liquid latex (very difficult to get flat and even, which is important).
But your best chance of good operation is getting a new pinch roller. Pinch rollers, alas, do not come in standard sizes, so you'll have to look for one that is made specially for your device... it's hard, I'm sorry. If you know exactly the size you need, you'll be able to do better, but still, you'll cry yourself to sleep.