Tyvek and Polycryo, which I believe is a similar material to a painter's drop cloth.

As for where to purchase: Z-Packs and MLD are just two places. You could look on Ebay also and I bet somewhere like Home Depot, Lowes etc would have painter's drop cloth which should be inexpensive. Another option is to ask around at construction sites for any Tyvek that will just be thrown away.