Oh, LOL I really hate to admit this but I am one frugal momma! At my work when we order supplies, we get all sizes of cardboard boxes sent to us. I just get an appropriate sized box, make sure it's all closed up (self-sealing boxes --> no tape!), and cut a hole in the side. I keep my babes in tubs so there's really no reason for me to get a "fancy" looking hide.
I do check regularly for mold but it hasn't been an issue yet (been using cardboard boxes for a couple months now). I'm sure there are more professional hides out there with bottoms but I do like my cheap alternative and the snakes like them as well [the cardboard seems to hold heat really well]!