I'm a Bottom Line fan
http://www.superiorthreads.com.
I always use it in bobbins, they last forever it's that thin.
If I don't want it as a top thread I use Aurifil, a lovely cotton thread - seams press really flat.
In answer to Homespun - I bought alot of Aurifil from a Canadian vendor. I've also bought from a gal in Massachusetts, can't remember her name. Try typing "Aurifil 50wt" in Google.
I see
http://www.redrockthreads.com has a good selection of the 50 wt.