The local Walmart here doesn't even sell class 15 bobbins anymore. No tag for them or anything, only class 66 in plastic (ugh) or metal.
I do have a class 15 bobbin that came with one of my featherweights ?!?! I keep it around to show people why not to buy cheap bobbins. It doesn't even fit into a class 15 bobbin case. If you really shove it does ( one guy did that, then I had to show him why -that- was a bad idea....)
Other than the vintage ones I have from various machines, I've only ever bought new ones from Sew-Classic, or one 10 pack from a regular seller on Ebay (terry something... I can't recall exactly). I have no FW bobbins here that don't work, so I'd second (third?) that Jenny's are good bobbins.