Thanks to Karen for this Frugal Baby Tip!
I use two 15 gallon Sterlite buckets. (like Rubbermaid, but smoother inside) $4.96 each at WalMart. One is upstairs and one is downstairs. When they are full, time to wash, that's just the right amount to fill but not overfill my washer.
If I had a one story house, I'd use one 20 gallon Sterlite bucket. dry. I like them best because they are shallow so they are easy to rinse out, when I do bother to do that. My 3 yr old can't get it open, but I find it easy.