callen,
It uses standard class 15 bobbins and 15x1 needles. You don't need to go to eBay for those. Most any fabric shop that carries sewing supplies will have them. Also any sewing machine shop will too.
JoAnns has them I know for sure. Just don't buy the 10 pack in the plastic box. Those are junk. The smaller pack in the blister card are OK. Singer sells them too if you find someplace that sells Singer stuff. Schmetz is one of the best needles to buy.
Go here:
http://www.singerco.com/accessories/instruction-manuals
Type in 15-91 in the little box and it will pop up a free .pdf file to download.
There you go, all set now.
Joe