You could buy them, and then use the top sheets later on...? When we used the cot, we used top sheets to put over Jazz, she never liked wrapping and was a winter baby so never managed to be warm enough, so I'd just put a sheet over her (SIDS safe sleeping recommendd making the bad at the bottom and only using as much sheet as you have to).

Actually, I've read on here members who just go ahead and buy single bed sheets, tuck the rest underneath, and you don't have to buy more when you upgrade from cot to single bed. Wish I had heard that before I bought cot sheets!