I'd agree that a .54 cal is a good size and it gives you many brands and models to choose from: T/C, Lyman, Investarms, Pedersoli and custom guns. In production guns, though, there is just the .58 T/C Fox Ridge Hawken or Renegade.
The likelihood of getting a custom gun built in time for this season isn't good but some builders might have one already built or a used one for sale. But we're talking a significant cost here, not just $400-500 for the T/C. If you really want a big caliber and the T/C isn't for you, call some local gun shops to see if they have any used ones available. There might be some .58 or .62 cals out there just waiting for you.