@Onebyone That's how I got started using SoFine thread, my friend's longarm couldn't wind a decent bobbin no matter what we tried, and her other machines couldn't handle the M bobbins, so we started buying prewounds from Superior Threads. I got my sidewinder and I think it does M, but we were so happy with the prewounds it was just easier to go that way.
The (used) long arm I bought last year (sadly, still not set up) came with some sort of Steampunk contraption bobbin winder that I haven't really looked at. All like cast iron and set on a wood board. I think her husband had been an electrical engineer or something and maybe built it himself. I'll make a point of looking at it to see if there are any manufacturers marks.