I agree with Jay, you want different threads. My preferences are:
Danville's Flymaster+ or Flat-waxed nylon for bass bugs and other spun deer hair flies
Danville's 140-denier for streamers (specially if I need a smooth underbody for tinsel or floss bodies)
Uni 6/0 and 8/0 for flies size 10 or smaller
I can't help you on the online question, I live a few miles from Stonerriver Outfitters that is where I get all my stuff.
Alberto