One source of greenscreen material would be the apple shake training book. You can also find some footage on the primatte website as far as i remember.
Note though that compositing is not only about green/blue screen. You might find that before you get into those sort of comps you will need to start out with roto for a few months and then cleanup and removals for a year maybe. Why not go out with a DV camera (not reccomended for green/bluescreen comps) and put together a few shots of replaicements (a new label on a moving bottle for instance), roto (look at the movie pleasantville for inspiration), track&patch jobs (wire and rig removals), etc. That might be much more relevant to your first job that a greenscreen comp.
I think that if you come to your first job with a simple reel demonstrating ability, you will move up the ladder faster. It really doesnt have to be much, dont try to go for length or complexity, 4 shots of 150 frames each is enough. Include breakdowns too. Remember, you are not trying to recreate the reel of someone who has been in the industry for years, you are just showing that you can do simple jobs well.