Check out shooters pro shop
they have some over- run 7 rum brass (nosler) you can neck up, or if you don't mind shooting factories for brass, you could get this load---
Nosler Custom Ammunition 300 Rem Ultra Mag 180gr Partition (Blem) 20ct
they had 8 boxes on hand when I typed this at about $45 a box
or this one
Nosler Trophy Grade Ammunition 300 Rem Ultra Mag 165gr Partition OVER RUN 20ct
that one was $40 a box and they had at least ten boxes