Safety 1st doesn't make sheets and they really don't recommend that any be used which is silly, but it protects them from liability from aftermarket products.
This is the play yard that we have and I believe the mattress base is a little narrower than Graco playards
We used the Carter's velour playard sheets like some of the BRU reviews recommended. They help with adding padding to the rather firm mattress base.
I might also consider buying an additional mattress to help provide more comfort for the safety 1st playard because ours just seems so thin and you can feel the gaps of the boards through the material. Safety is a big concern though because the mattress would need to fit flat with no gaps and this playard mattress platform seems to have a less standard sizing.