There are lots of other sources of lead bullets but mastercastbullets.com combines low prices and highly accurate bullets in one stop.
Almost all cast bullet companies are very small and most are no more than a couple of guys. Vacation time in August or hunting season in Fall/Winter can close them up for a while.
I have always found that simply leaving a message and a phone number gets Mike within a day.
After mastercast, I look to Missouri Bullets, Bootheel Bullets, and Penn.