My Team has never used treads, but I've done a lot of mechanism scouting on a lot of robots from a number of years. So heres what I see the most.
Treads? The most commonly two used have a Roughtop and Wedgetop tread
Look at this thread
here for a whole lot more discussion on it. Conveyor belt materials are the material most used.
Where to Buy?
McMaster-Carr Is popular, reliable and VERY quick. A lot of information related to treads (from the tread itself to the material to lacing it) can be found on pages 1091 to 1096 of their website. (They dont like links so just search for one of those pages)
Size? I've seen anything from 1" all the way to 4" thick treads, I'd say theres a good range of 1.5" to 3" but it all depends on the speed, manueverability, and friction you want to make with the ground. The Length usually is long enough to go around as if their was wheels (once in awhile you see a half track design, only half the robot having treads, the rest with wheels)
Lacing, I dont know too much on this but choose a system to replace easy, treads tend to wear down over practise, regionals, and the championship. McMaster pages 1094-1096 had appropriate materials for lacing.
Check out this site for some pictures of robots using treads, it may help out a bit (wheels are there too)
http://firstrobotics.uwaterloo.ca/wheelsgallery.php as for teams that have used treads successfully in the past, Team 180, SPAM is the most reknowned for using treads effectively.