you can buy it on ebay by the foot
http://cgi.ebay.com/Tyvek-Ground-Cloth- . dZViewItem
i bought some and use it for a tent footprint. great stuff. it's VERY rigid when new, i was told to put it through a few washings to soften it up.
ive heard of people having good luck going to construction sites and asking for a big scrap piece, explaining to the foreman what you're doing with it (camping) and slipping them a tenner.
also a few people out there that have made a living making things out of it such as this guy
http://www.goodlingoutdoor.com/Poncho.html
i kind of want one of the poncho/tarptent combos lower on the page. great weight savings in good weather hiking areas.