I'd have more info for you if you were interested in 400+grainers. Whytesleatherworks offers a 300 grain .458 he calls the 45-60. Not sure how soft you want it.....he offers them in about BHN9 alloy.http://www.whyteleatherworks.com/BigLube.html...........................Contact this guy, and see what he can do for you. I bought 750 slugs from him last time and am quite pleased with them.
I have a Lee mold #90373 coming for 340 gr. in .457.... That likely means they will drop between .457 and .459 I would think.... I'll let you know once the mold gets here and I have some time.... I plan to cast them in 1:100 alloy (WAG maybe BHN 6-7).... I'm not going to be doing them for sale, but if they work out size-wise maybe you could then find somebody willing to make them for you.... FYI, just the cost of the lead is 11 cents a round plus shipping (from Rotometals).... No interest in casting your own?.... or just not enough time (I know the feeling).... Bob