The answer to who supplies the best one is either CNCdrive (who make them) or, in the UK, CNC4YOU. CNCdrive have an active support forum available if you need any help. I use a couple of the big brothers of the UC100 (in fact, UC300) and they just work. Never had any problem with them.

You will also see UC100 advertised on various other web sites. Be aware that there are a lot of fake versions around that nearly but not quite work. There are also a lot of "USB motion controllers" available via eBay and other places. They may work as advertised. I don't personally have any experience of them, and there is a fair chance that there is no technical support available from the supplier. On the other hand, there seem to be some available for less than a tenner, so maybe it's worth a gamble! Double-check their power requirements if you do go down that route - the one on my screen at the moment needs a 24V supply, apparently, and I have no idea if this is available in your system.

You can also find "USB to parallel port" converters. These are designed to work with printers, not CNC systems, and are very unlikely to work in your system.