The one thing that link doesn't do is tell you how many bobbins you get for $6.95. Since class 15 is a standard bobbin size, I am thinking that JoAnn Fabrics, Hancocks, and other places (maybe even Walmart?) would stock the plastic ones.
Class 15 bobbins also come in metal, but those are typically for the vintage machines. If your machine came with a plastic bobbin, you want to stick with plastic bobbins so as not to damage the bobbin case.
I would check locally for these bobbins before ordering online, as my bet is that they are widely available (because they fit so many machines).