Conicals for 58's are falling by the wayside these days. LEE REAL's are an easy fix if you cast your own. Otherwise, check with Track of the Wolf and see what they offer. You'll see hollow based minies in their listing, but I'd stay clear of them if you plan on using more than about 80 grains of powder.
As for the odds and ends market, find them as you can, I'd sure keep your eyes open for any 58 caliber Hornady Great Plains conicals you can find. They're super accurate and deadly on game, but sadly out of production TC Maxi's are fine, but I got quicker kills with their Maxi Hunter. More to watch for.
I wouldn't have any qualms at all about using plain old round balls for pigs. They weigh around 100 grains more than a 50 cal ball, and even in pure lead they penetrate like crazy. Harden them up with an alloy along the lines of wheelweights and they'll whistle through about anything you poke with them.
As a "just fer instance," both of my Big Boars really seem to thrive on 100 grains of 3f or 110 grains of 2f with round balls. They'll take more 2f than that without any loss of accuracy, but you'll really start to notice it on the back end when you hit 120 grains of 2f. My more or less standard load with all conicals is 100 grains of 2f. You most certainly notice it when you light one off.