Aluminum is the tried and true material for most FIRST robots. Remember though, until this past year, so called "exotic materials" were band. For robot frame construction, aluminum is still the way to go. Its readily available at any metal supply store, and relatively cheap.
Titanium and carbon fiber can be quite useful for teams that can afford it, and have to tools to shape it. I doubt any team would ever use titanium for their frame, seeing as it needs to be welded in an argon atmosphere. Titanium shafts are a good replacement for steel shafts, seeing as it is quite a bit lighter, by volume. (roughly half that of steel) Titanium is quite a bit harder to machine.
Carbon fiber is a whole different story. It can be quite useful for grippers and manipulators. We(1114) used it last year for our big ball gripper. It was an incredibly lightweight solution to designing our ball gripper. Carbon fiber has a density roughly half that of aluminum. It is also dangerous if the vapors are inhaled when machining, so teams should take precautions when working with it.(filter masks, work outside, and of course safety glasses)
Both titanium and carbon fiber are available at
http://www.robotmarketplace.com/
although I'm sure there are cheaper suppliers for the stuff
mcmaster-carr also has titanium, in the raw materials section under metals
www.mcmaster.com