I only used fitted sheets and had about 5 for DD. I bought a couple of cute sets (one from Target, the other Big W) and then bought just fitted sheets in plain white from Target. I always got them cheap at the baby sales. I got a hand-me-down set too.

My only issue is I have a Boori cot and the normal cot sheets are quite small and tight on my cot mattress so I wish I'd spent more on the sheets designed to fit Boori cots.