Our Wal-Marts don't carry the class 15 bobbins anymore. Not in metal anyway.
I ordered the same metal class 15 bobbins that Jenny carries from the supplier here in Canada (their spreadsheet refers to them as "top quality"), and have had no trouble with them at all in the 15-90 here, and no complaints from the lady who bought a bunch from me with the 15-91 she bought off me. Maybe I just got lucky?
The ones Jenny's recommending for the model 15s is the plastic ones now?!?! (That's what the 2518P is)