I asked this question too. I went to the dollar store and found little baskets, but they all had solid bottoms, which I thought would hinder air flow. Also, none were very tall, and I was afraid the chicks would jump out....Then I realized my incubator wasn't very tall, and that the shorter baskets would have to do. I did find some plastic coated wire basket with open wire bottoms at the dollar store, in a couple of sizes.
I never have worried about a solid bottom..... I put shelf liner and a paper towel on them so the chicks will not slip. Mine is forced air so I think the sides and tops are going to give PLENTY of air circulation.