T.E.
I'm not sure of the dimensions, but I would venture to find some fabric that I love, measure it as wide and long as the mattress, add 6 inches all around, and cut it, hem it, and sew elastic to the corners (after rounding them a bit) You could try this with a cheapo, old, sheet first just to make sure it works, etc.
This is what I would do, though, so that I knew I could have any sheets I wanted, including velour. :)
Teri
ps, I love Ikea!