There's a brand on Amazon called "Babygoal" that sells just the waterproof cover, so that you can use it with prefolds/flat fold diapers. It also has a pocket that you can insert a liner, if you want to. Here's an example.

I bought a pack of 6 and will be trying them out with osocozy flat folds soon. (I'm due in 3 weeks but plan to use cloth after the first month or so!)