The IFI legacy site has the compiler for Pbasic and the default code for the controllers. It also has the default code reference guide which is handy.
My freshman year was the last year FIRST used the BASIC stamp controllers. I think learning to program with Pbasic first made C a lot easier the next year.
I had never programmed before and was the only programmer on my team all years of high school. The best part about the Pbasic controllers is that Rob Bayer wrote an amazing emulator,
Roboemu, for them so you don't need to have a robot to learn to program the controllers. This made learning to program so much easier.
A few weeks ago, I went back and wrote some new code for our 03 and 01 robots both of which still have the BASIC stamp controllers.