I don't know where you are but if you were able to order a Flemish string, you should be able to order B-50.
3Rivers has it, so should any of the better trad vendors. Chad is a dealer for B-500; same stuff as B-50, just marketed by BCY rather than Brownells.

Tell you what, if you're in the states I'll send you a couple short spools that I have. The blue doesn't have a lot left but the green has enough to at least get you started. Drop me a line at [email protected].

Guy