[shilala]

I get large amounts from goosebay lumber, but I'm guessing you just want some 1/4" slats or something? Rockler is good for that. They're expensive as hell, but for all you're looking at, it's gonna be much the same anywhere.
If you need something made, pm Forrest (golfnut). I'm sure he can make whatever you need to your specs, and it'll likely be around the same money, depending on what you're doing.