GunBot.net is my usual source. You don't see their name come up that often. While GunBot is not all inclusive of retailing vendors, they do have a fairly broad selection of suppliers.
You might look at the burn rate chart and find something similar in pressure and velocity. IMR 4831, one peg above Hunter, happens to be on the list of available powders at GunBot, not sure if it fits your needs.
Good luck to you.
