Can anyone give me a good,(re: cheap), source for 50-90 ss brass? Or tell me if there is something else I could form them from? Thanks in advance for any help.

Just a guess... you might be able to form from .50-110.  50-110's available from Starline.  Hope this helps.

You might want to try the Old Western Scrounger in No Calif. his web site is
www.ows-ammunition.com    He has a lot of everything in old and obsolete
calibers.

Gun Runner

try: www.buffaloarms.com/ for 50-90x2 1/2 Sharps brass.