Yes they have sheets for the Pack N Play cribs and yes they launder them separately. You can ask for a fresh one if you like rather than use the one on the crib. At least you can at the resorts, I don't know about a cruse.
You can definately buy additional sheets in PNP size at most stores including wallmart and babies r us. I always brought a couple on vacation with us so dd would be familiar with them, laundered in our regular detergent. I also knew I would have one and that it would be clean.
Here is a link to the pnp sheets, fyi:
http://www.amazon.com/exec/obidos/A...26814/sr=11-1/ref=sr_11_1/002-0236783-2481662
TJ