Didn't realize that .50 BMG was a victim of the lunacy too. All I could find were places that "used" to have them. Wonder if a call to Barrett might get you some leads? I'll keep looking around. Good luck.
With the way this country feels about black rifle's now day your going to need all the luck you can get on finding any bullets for that 50BMG.
I sold my Barrett 50 & and my brand new Barrett 416 that I just got this year due to you can't find bullets. I can get the ammo from Barrett but for 20rds cost a little over $400 bucks match grade. So I sold my two long range rifles that I loved and told my wife I am going to get a little deeper in to trap shooting this year and give up long range shooting.
I just talk to a guy that sells rifle reloading stuff tonight and he is telling me anything to do with rifles your going to be lucky to find, the reloading stuff for shot guns is going to be letting up some here before long and your going to be able to find that stuff easy.