S.
Like the other two responses, I think Craig's List is your best chance to find something. But another option is a consignment store on Inwood Lane, just north of Forest Lane. I can't remember the name right now. They often have a good selection of beds, but they don't carry mattresses. Also, keep your eye out for garage sales. You'll get your best buy there, but of course, it's hard to know what you'll find!
Be willing to consider a daybed. They are great for little ones, because you have a built-in "rail" on the side with the back, and then you can just put a single rail on the other side for safety. If you get one with a trundle, you could let your daughter sleep on the trundle (without raising it) until she is used to that size bed. Good luck!