R.C.
It's not super-inexpensive, but it's good for long term: we bought a twin mattress from Sam's Club, that would eventually fit his "real" bed, and simply laid it on the floor (so he wouldn't have as far to fall) for a while, until we knew he wasn't falling out of bed. The mattress was between $150 and $200, i think... But I do think that Walmart sells cute little toddler beds for around $40?