One comment, before I launch into gearbox design: I doubt you're going to be able to find a system smaller than the BaneBots planetaries. They are some of the smallest and toughest premade systems I have seen... But anywhoo:

As for gears, you can try McMaster (I haven't used them, but who knows?), or you can try to find a local distributor of Martin Sprocket (the gear company that we use). The two sizes that seem to work for FIRST use are 20 dp, and 24 dp. I personally stick with 20, as the tooth size is larger, which helps the bot take the load of defensive pushing matches. Next up, there are two options that work best for attaching your gears to shafts: The first is with keyway, which requires a broach into the gear, and a keyway to be cut into the shaft. The other is with hex shaft, which, depending on your team resources can be either excellent or terrible. The only downside I see in hex broach is the cost for the broach and the shaft. However, it aids in part change time.

One of the things I can suggest to learn more about making a gearbox for your team would be to walk around whichever competitions you'll be going to, and look at team's drive systems. Ask them how they designed it, and study their system. If you'd like more of a conversational education in gearboxes, feel free to AIM or email me, I'd be glad to discuss them whenever I'm online.