The Lifeline Jungle Gym is very similar to the TRX for about 1/3 of the price. I've never used either one so I can't say which is better but I have seen a lot of recomendations for this one as well. It's adjustable and there are quite a few youtube videos that show how it can be used.

For that matter get some cordage or webbing at a local REI, knowledge of a prussic knot, and some tubing for the handles, you could make the thing for well less than $20

Prussic knot: http://www.youtube.com/watch?v=P0d8-Fxdx8Y&NR=1