Limited run. They only make so much per year. You can buy it off Federals website.
Try the 123g SST if you haven't. Im not a fan of SST's at speeds above 3k but it works well below. It shoots better than the AMAX in mine. I have some 100g Partitions bullets I bought to see exactly how fast i could push them but haven't had the chance to load anything in a while.
Both of my Grendels shoot the 123 SST's very well and I have taken Sitka Blacktails with them. With that said, I haven't purchased any lately but have heard some recent runs had accuracy issues in some guns. The issues I've read about probably won't be an issue in hunting situations. More info is available on the 6.5 Grendel Forum.
For info, factory 123's are averaging about 2460 at the muzzle in my 18" gun and 2225 out of my 12" tube.