I know the OP is looking for a good .54 bullet that doesn't need casting, and I agree they're getting rare. It's a shame. I remember a time when .54 was considered ideal for whitetail. I shot a .54 TC Hawken for decades.
The best option I've found is Lee R.E.A.L. cast lead bullets. I started over a decade ago. I bought a Lee bottom pour pot and the mold and found some $1/lb lead and went for it. It wasn't a hard skill to master, and for the first few years I spent less than an hour a year casting up enough balls for the next year. In a couple of seasons, I paid for the investment with the per round savings over shooting CVA Powerbelts.
I next started casting .50 R.E.A.L. for my sons' smokepoles and after that, I just kept going. I'm now shooting a bunch of pistols and centerfire rifles with cast lead as well.
My latest casting project: Lee R.E.A.L. 50 cals out of an inline ML. I'm getting Minute-of-Deer accuracy at 100 yards and I'm gradually shrinking the group.
