Kid Bed Help: research?

I still love Craigslist like crazy, but finding a mattress to fit Fiona Pie's "new" loft bed is proving to be more complicated than I'd imagined. The thing was built to accommodate a college dorm-sized twin mattress...that is, extra-long.

Approximate dimensions are 36 x 88 inches. If anybody knows of a local, affordable option, would you give me a clue where to look? I'd greatly appreciate the help finding a solution!