I buy my (polyester) bobbin thread from Doris Darby at
embroideredjust4uI buy the 5000m cones (both white and black) and prefer to wind my own bobbins. It's a fine smooth, lovely polyester HT 60s weight bobbin thread.
I know that lots of others buy the pre-wound bobbins in bulk. Doris does both sizes - there is a
chart here to check which size of prewound bobbin your machine uses if you want to try the pre-wounds.
For a spun polyester (matt) bobbin thread, I have cones of
this from Empress Mills. I tend to use this one when embroidering with Rayon 40s top thread (Madeira Classic rayon), and also use this spun polylight for both top and bobbin thread for matt finish FSL (and to test stitch FSL designs).
For most embroidery that I do, I use polyester embroidery threads (Simthread or Kingstar) and I prefer the HT Poly bobbin thread from Doris above (or Empress Mills) for that.
I tend to buy it from Doris because I usually top up on
Organ embrodiery needles, my go to stabiliser,
polymesh cutaway and any
Simthread colours I may be running low on.
I don't often replace the Simthreads as I mostly use
Kingstar which I get from Somac along with their temporary adhesive and some other items. You can also buy
bobbin thread from Somac - they call it underthread. (It's a spun polyester (matt) and it's okay, but I prefer the other 2 I mentioned above).
Quick note.
Some machines offer a different bobbin case for bobbin threads that does not have the same properties as the 'brand' used to calibrate the machine's bobbin case tension, eg. the red and yellow dot bobbin cases for some Janome machines. On other machines, and just in general, when you regularly use different types and brands of bobbin threads as I do, then it can be useful to buy a couple of spare bobbin cases and set the tension yourself (if needed) for each type of bobbin thread. As my main embroidery machine is a combination machine, I keep a bobbin case for sewing, one for HT poly bobbin thread and one for the matt spun polyester. It just makes life simpler and reduces the need to adjust the top tension. (If needed I still adjust the top tension when working on very thin or thicker than usual fabrics but that's a different topic).