I see someone else has already mentioned MidSouth assuming they still have it. But check all the usual powder suppliers it one has it others will be shortly behind. But if you can't find it I would not go nuts looking for it, just load with what you can get
just speaking for the .260 Remington which uses a 6.5 - .264 dia bullet. Never loaded for any of the other 6.5 cartridges so the milage may vary.
I have found several powders which have wide load nodes which can get sub MOA out to 800 yards which is my limit for testing at the moment. Varget, H4381SC, RL17 and IMR 4350 coupled with the right bullet at the right seating depth have worked well for me with bullets ranging from 120 - 142 grains.
My rifles both like the Varget, SC4831, and RL 17 best with the IMR 4350 only working 120 and 123 gr bullets. However it delivers those bullets with better than good numbers. In my opinion 107 grain bullets are worthless for target shooting so I never really tried very hard to find a load or powder for them.
Ok now this is just second hand info but a shooter I know had zero luck with H4350 in his 6.5 Creedmore. He offered to sell me the biggest part of a 8 pound jug at a good price. I declined because what I have on the shelf works for me. He sold it to the next guy he asked and the guy did a happy happy joy joy dance
Not sure if this helps but good luck