I'd say it's a Schwinn Manta Ray (are they 26" wheels?)...but without any specifics I can't be sure.

Schwinn made the stingray back in the 60-70's and it was a HUGE hit. In fact, they're coming back now. They also made bigger versions for fat kids (ok, older kids) that were called (I believe) Manta Rays.

That crankset looks to be Schwinn for sure but the seat is wrong, it would have had a banana with "sissy bars".

Search ebay for Schwinn Manta Ray and see what you get. If there's nothing there search for stingray and see the little version (20" wheels). Also, search Google and find a devoted fan site. Or, go to Schwinn bikes online, I guarantee there will be something about them on there.

Try this link: http://www.firstflightbikes.com/stingray.htm

Hope this helps.
PJ