I got mine at Walmart. The inner one is plain white and hangs inside the tub; the outer one is patterned and hangs outside the tub. Both are cloth and I wash the inner one every week. I have a couple so I'm never short.
My local Walmart has a very good selection of the outer ones. Don't be swayed by curtains that come with magnets at the bottom corners. They will stick to the washer and dryer walls and rip out in a few washes.