Did you give the trade blanket forum a look over at Dougs WS? I know someone had a few "BOs" for sale there.
If all else fails see if you can find some 300 grain Remington .458 SJHPs for sale. They are designed spacificly for use in the 45/70 so are better suited for higher velocities. I know Cabelas sells them both OTC and MO, and the Rems have been proven by meny to shoot excellently and perform equally as well on deer. The sabot of choice used with them has been the Harvester BCR and the orange MMP sabot.
I shot both the .458 Rems and the .458 "BOs" out of my 10ML-II and they both gave excellent accuracy. I used Harvester BCR with both as well as H-4198 around 65-66grns.
Midway, NSS, as well as others also sell the 458 Remingtons. seeing as it is so close to firearm deer season, I can see why you are having so hard a time locating certain types of bullets for use in MLs.
Good luck and remeber this for next years season.