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Slats or sheet goods would work just fine. You can probably by 1x3 or 1x4 spruce pretty cheap from a hardware store - the same place as your 2x4s. I recently built my son's twin bed, using the same stuff. They were 1x3s, and I placed them about 3" apart. He loves the bed :D
I believe I used 8ft 2x4s.