I am looking for help in finding a sheet to cover my couch.  The sheet
(I prefer it not to be a slipcover, but just something to throw over
the couch) needs to be waterproof on one side and fabric on the other
side.  I think something like that is sold as a picnic tarp or the
like.  It must be washable.

The real kicker is it needs to be at least 11' x 6'.  (Longer in
either direction is fine, within reason - i.e., not 20' x 20' or
something.)

I want to be able to buy it online or by catalog, and it shouldn't
cost more than $50.

Much gratitude if you find such an item for sale :)  (Auctions are fine too.)

Here are some rubberized fabrics that might be useful if you'd like to
sew this yourself:

http://tores-tex.en.alibaba.com/group/10217085/Waterproof_Cloth.html

http://www.4onlineart.com/Fabrics/Decorator-Fabrics/s_6901.html

http://www.joann.com/catalog.jhtml?CATID=54498&PRODID=34522&AID=10273743&PID=1212837

You might be able to find something at a store that carries camping
supplies. Waterproof fabric tarps and ground cloths are often
available in the tent department. Some might be large enough for your
purposes.