A round lead ball that weighs exactly 1/28th of a pound will be.550" in diameter. A 28 gauge bore will also measure.550". Your .56 smoothbore is .010" larger than a true 28 gauge, although shotguns are sometimes "backbored" to a slightly larger diameter and your .56 may well be within accepted parameters for a backbored 28 gauge... I don't know.
In any event, I don't think you are likely to find a 28 gauge round ball mould, listed as such. However, Moose Moulds will make a top-quality .550" round ball mould for you with wood handles and a sprue cutoff plate, I think in single or double cavity. I bought one of their single cavity .550" moulds and found it was very well made. Not cheap, though... I think it was right at $100. They are good folks to deal with, too. Cordial, efficient, and no nonsense.
You can Google "moose moulds" to find their website. They have a few other atypical and "normal" roundball sizes, also.
I have no personal experience with Tanner Moulds or Larry Callahan, but I understand they both make round ball moulds in virtually any size, and I've never heard anything bad about either of them.
There was a recent thread on the ALR forum about a new outfit that makes gang moulds. I'm pretty sure they offer one in .550". I'll try to look up the link when I get on my computer (using the iPhone right now), or you can go to the ALR forum and search for "gang moulds."
One other option is to just buy pre-cast .550" balls from Track of the Wolf. They usually have them in stock. I think their balls are cast by Rush Creek.
Good luck with all of this! Be safe and have fun.
Notchy Bob