Basically anything that is firm and snug in your crib. We have this one http://www.amazon.com/gp/product/B004044LD4/ref=oh_details_o04_s00_i00 and it fits great in our crib. I was actually kind of worried it was too big but its in there and VERY snug. haha
As long as it's firm and snug. My sister used one from walmart that was like $40. I opted for a Sealy from Target b/c we had a giftcard and I wanted a brand I was familiar with, mine was $69. But they're all pretty much the same. Firm and covered in plastic lol.