Barnes has 458s starting at 250gr and Hornady just added a 250gr tipped MonoFlex 458 for sale in Bulk. Both are made for 45/70 speeds. Speer has 475s starting at 275gr but they set very deep in the sabot. Plus all the Lehigh/Knight options in 458 make it plenty flexible IMO.

I like the 325gr FTX for a semi affordable 458 "pointy bullet option. I get them at the gun show for $36/100. Ive shot them at sub and smokeless speeds with no complaints.

Last time i got sabots from Knight, both 52 options were about $9.50/50 which isnt that bad IMO. They are very tough sabots too. Your shoulder will likely say that's enough before the sabot does.