Well as an alterative view point, B50 isn't even useful as a tillering string because you have to bend the bow so much to allow for the stretch. I've braced a bow being tillered 4" and more with it and literally watched it stretch down to nothing. Don't get your fingers caught in there! Similarly, it robs cast and increases noise, wrist slap and handshock. Fortunately if you craft your string grooves and string loops correctly, you don't even have to re-inforce for FastFlight, even on woods as soft as elm, even yew if you are careful, in my experience.
I personally would go back to FastFlight. At around $23 for 8900' the last time I checked it's a great value, compared to $8 for 4200' of B50 or something like 450+ at $50 a spool. I think I figured it costs about a quarter (25 cents) of material to build a FF string compared to about 16 cents for B50 way back over a decade ago when I was trying to decide what to use. I tested both and when I first tried FF was amazed at the difference between Dacron and low stretch materials.
For the bowyer/hobbiest it's just free performance, and particularly pronounced on bows with lots of early draw weight, and/or really long bows like elb. After you've built a few bows, performance becomes increasingly the goal, after all the other criteria are satisfied. Why build, and draw, a 60# bow when a 50# can make the same cast? Why put up with that low "hummmmmm" as a bow continues to vibrate after the shot, not to mention the handshock and wrist slap.
And I bet if you find a vendor with B50, they'll have low stretch as well, probably FastFlight.