My friend's longarm couldn't wind a decent bobbin to save its (or its users) lives! At that point we could pretty easily find L bobbins, but no M in the larger Seattle area. We ordered prewounds from Superior Threads.
https://www.superiorthreads.com/shop...bbins/c/60-215
It was when I began using So Fine as my thread. We were initially leary of the cardboard bobbins but never had an issue related to that. First with quilting bobbins, then as sewing thread as well. That was a few years ago, Superior changed ownership, I don't know if they have as many specials on trying out bobbins as they used to, but if I/we were buying cones we'd slip in a couple samples at the same time.