For my first baby 13 years ago (!) We were struggling for $ but I wouldn't compromise on 100% cotton so I went to an Op Shop and bought some great sheets that felt like ex-motel sheets.... very thick, you could tell it was 100% cotton, probably 700+ thread count too (I love my 1000 count sheets these days!) and my mum made them up into cot sheets. I soaked them in Nappy San first. So that was dealt with for about $10 from memory. Got about 4 sets and that was enough... people bought us a few extras and we were given a few extras... but the Op Shop ones were the nicest. They fitted perfectly too because mum was able to measure the mattress.

I agree with BW too.... sleeping in synthetic feels so humid and yuk... just like a sleeping bag.