It wasn't a sheet. It's a couple yards of white jersey that I bought specifically for still lifes. Jersey doesn't wrinkle the way sheeting does.
The next time you go to Wal-Mart, go into their fabric department. They always have a table or two loaded with fabric for one or two dollars a yard. That's where I got the jersey. Look for basic colors and pick up a few yards of each. Black, white, and cream to start. If you're not sure which fabrics will drape best and if your mom is good with such stuff, ask her to help out.