I think that Mid-Kansas Cast Bullets is now offering a 50 cal blackpowder bullet. I don't know if it's on their website yet (mkcb.com, I think), but it's in their paper brochure.
You're right though, about 50 cal bullets being lots harder to find than 45 cal.
You're right though, about 50 cal bullets being lots harder to find than 45 cal.