I have made my own balls since the early 80s, that way you have what you need...
I make .530s for my .54 and .390s for my .40...
I have both Lee molds and Rapine bag molds for my .54, only a Rapine for my .40...
If you don't want to make your own, order from Track of the Wolf...They even have .526s for .54s with a tight bore or for that quick second shot without swabbing...