1. You can get cameras from anywhere, even a crappy webcam. Or you can try places like dealextreme.com, they have a few for cheap. But you're going to want one that has a mirrored output as well as a wide angle and you'll want CCD, not CMOS, if you want any sort of quality video. And then there's the issue with infrared LED for nighttime use and it goes on.
2. Why USB? You don't need to record the video (unless you want to), you just need to view it when reversing, right? So just hook it up via RCA and you're good.
3. There are converters for RCA-USB but they're not going to use all the available functions of USB. Like you won't be able to do data transfer for recording unless you have a capture card and you also won't be able to power the camera that way since RCA doesn't have a 5 volt rail for power.