BBB sells a cotton bag for $8 that is 24x34 or something. It shrinks to 22 x whatever after using once.
It works ok, it not too restrictive, but not as good a the older synthetic bag they used to sell. The new synthetic is woven too tightly, and your work will not drain.
Supposedly paint strainer bags will work. (they are cheap at Home Depot).
I just ordered a wilserbrewer bag.