did anyone see ben snowdens foam pit on ridebmx.com? it was pretty much made entirely from scrap pillows and cushions etc. ya, i would deffinitaly just look for old couches and stuff at junk yards, garage sales etc and use da cushions.
Post up flyers at your local supermarkets or whatnot saying "hey if anyone's got a couch or pillows they're throwing out, I can pick them up for free" etc. with your phone number. Other than that, yeah, rip it out of car seats at the junkyard, pick up old couches on the side of the road, do anything you can.
I have found a web site that sells the gymnastic foam, looks like it ranges from 0.88 cents a piece to 2.09 a piece. I'm sure thats gonna add up! Heres a link. shttp://www.ten-o.com/d/375,1_Gymnastics-Foam-Pits-and-Pit-Cubes.html