Different brands are available in different ways -- I buy directly from Superior, Bottom Line is a line of threads as is So Fine.
https://www.superiorthreads.com/
Different people have different experiences with prewound bobbins, when my friend got her long arm, we couldn't wind a good bobbin to save our life and that's when I started going to Superior, first to buy prewound bobbins and then just to buy my thread.