I figured I'd have a go and do some research on where people get foam pits for heavy tricks.  I found this forum, and basically building a foam pit would be pretty pricey, as the type of foam you need costs more than regular foam.  They also are saying that you really need to upkeep your pit and plan to have some kind of protection against whatever weather you may face, which is common sense.

Well anyway here is the forum where I found all the info, check it out if your interested http://bb.nsmb.com/showthread.php?t=77973