The best forum for R/C Bikes can be found here
If you're looking to go onroad, I reccomend you start with a 1/5 bike, like the Thunder Tiger. 1/8 bikes are much more finicky, and excess HP just turns into flips.
For off-road, you may be able to pick up a cheap 1/4 "Ricky Carmichael" Radio Shack off eBay. There are a few good mods available for it.
If you're looking for an online shop, I'd try this one