The reason why stereo USB mics don't exist and/or are not good, is because stereo mics in general don't really exist. In all professional recording applications, an array of two (binaural) or sometimes more mics are used to create the stereo field. There are stereo microphones out there, particularly condensed "Mid-Side" arrays, but keep in mind, these are actually two mics inside of a single capsule, and typically require a special adapter.
A few common mics- I know Shure makes a stereo mic, but it is quite expensive. Probably the best value for a low price out there is the Rode Stereo Video Mic, but this isn't intended for studio recording. As I discuss below at the end, there are also handheld interfaces that have built-in stereo arrays (typically XY, or in a few cases, Mid-Side).
There are a number of approaches to stereo recording with two mics. This depends partly on the type of microphone in use (large diaphragm condenser/LDC VS. small diaphragm condenser/SDC) and the type of "polar pattern" or directional response of the microphone (cardioid VS omni). Shure (mic manufacturer) has a nice webpage on this.
So... two mics? Low S/N? How do?
You're going to need mics that use XLR cables, and likely condenser mics at that. XLR cables are like super-duper 1/4" cables- they protect the signal from noise and hum from electromagnetic emissions and other issues (which 1/4" and 1/8" cables do NOT do), and can provide power to the microphone to power circuitry inside it.
To get XLR analog audio data into your digital sound card, you're also going to need an audio interface. Focusrite just released a new generation of their excellent entry/mid-level audio Scarlett line. I honestly wouldn't go for an interface any cheaper than that unless you want it to fall apart or have poor S/N or other issues, but you are welcome to check out local thrift stores, antique shops, tag sales, etc. in your area- you might just find a functioning USB interface: just remember, it needs two XLR ports to do stereo!
As far as mics, it depends on what you are recording. For vocals or such, it is much more common to use LDC's than SDC's. LDC's respond slower, but have a warmer sound and fuller low-end. SDC's respond very well to high, fast, and loud, and have a very clear and accurate high-end, but often lack in warmth, especially at entry-level models. SDC's work excellent on pianos, drums, guitars, small percussion, and quite well on all types of instrumental ensembles. LDC's work best on voices, woodwinds, low brass, low strings, and instrumental ensembles.
If you just need a general all-around solution for things like, recording a small group quickly with just two mics, then maybe a Rode M5 pair would suffice. A super-cheap LDC pair might be a set of Samson C01's (pretty much a repurposed USB mic). For better results, the AKG P220 or a Rode NT1-A will do, but getting a pair will be considerably more expensive. I'd recommend recording pop-type vocals in mono with an NT1-A or such. It is, in my opinion, the best value LDC in that price bracket.
This stuff is expensive if you are not into pro audio! But, it is an investment that must be made if you are after stereo and at least passable quality.
Another possible approach is using a hybrid interface-mic design, like the Zoom H line. These handheld interfaces can actually be used as an audio device when left plugged into your computer, and have a stereo array built in (not to mention do quite well in the field away from your computer, whilst fitting in your pocket, unlike traditional interfaces, which require power and do not fit in pockets). This might be your best bet for the most affordable option, but again, these are SDC's and there is only so much quality you can fit into a smaller box.