I forgot to say, Michelle, you can find vintage bobbins online (eg. ebay) fairly inexpensively if you browse around. If you search 'featherweight bobbins' you'll find the higher priced ones, but if you do more sleuthing you can find them cheaper. The FW bobbins look similar to class 15 at first glance, but they are wider and thinner, so it is possible to identify them from a picture. You can always ask for measurements if you're not sure.
Here's a lot of 17 FW bobbins with a bunch of class 15 coming along for the ride. If you don't need the class 15 it's more expensive than buying new, but less than properly advertised vintage FW bobbins. Plus you get two cases:
http://www.ebay.com/itm/LOT-of-33-ME...item35cc1912fb
Only 1 FW bobbin here, but a nice buy for class 66 bobbins. I'm guessing they are ca. 1970s by the graphics on the box and also the mention of Singer 600 series:
http://www.ebay.com/itm/Three-Bobbin...item4ac9c4429e
I found those after just a couple minutes of searching "metal bobbins". You can find better deals if you spend more time looking.