We made one recently from instructions I found online and a video I found on youtube. It isn't as good as the one at the gym, but my L4 son practices on it and it was really cheap to make.
The other thing you could do is talk to coach or some of the parents of older gymnasts. We bought ours off a parent whose son was a level 8 and didn't need it anymore. Lots of mushrooms get passed down that way at our gym. You can also check craigslist.
Thanks for the help, I have considered making 1 but we just moved and I have so much to do with the house that idea is right out. I have checked Craigslist a number of times and they come and go.
I figured the best bet was to find 1 through other families whose gymnast has mastered it and has no more need of it.