I was curious, and investigated a little. It appears Lee Precision makes a six-cavity "gang mould" for .520" round balls, evidently marketed exclusively for Track of the Wolf. These are listed in Lee's current catalog, although, predictably, they are currently out of stock. I could not confirm that Lyman, RCBS, or Saeco produced moulds in .520". I don't doubt that they did, but I just couldn't confirm it.
I know the OP wants to do his own casting, but until a mould turns up, Hornady does make swaged round balls in .520". I found a few of those for sale. I think the current shortages apply more to the common sizes of balls than to the "outliers," like .520" or .480".
It is a shame and a pity that the mould manufacturers have discontinued so many sizes. They all compete with each other by making the same limited range of ball diameters now. If they would be willing to listen to customers and break out of their ruts, I think it would be financially rewarding for them. There was a lengthy thread on this forum a few months ago about .526" balls. I could use a mould in that size for my .54 caliber rifle. Lyman used to make a .526" mould, and I just discovered that twenty or so years ago, Lee was producing a .527" round ball mould. Chances are, Lyman and Lee both have those cherries kicking around the factory somewhere. When the current demand slows up, I think they would do well to reintroduce some limited runs of moulds for "obsolete" round ball sizes.
Just one old dude's opinion. If nothing else, it ought to bump this thread back to the top for a bit. Good luck finding that mould!
Notchy Bob