Hancock fabrics, Joanns (they show 3 on their website), likely Hobby Lobby, they seem to be pretty commonly available nowadays. Make sure it will accommodate your type of bobbin, I've heard some people say their bobbins don't fit.

If you can get a coupon and have a retailer nearby you'll save some $$. Good luck.

Here is a link that might be useful: one at hancockfabrics.com