Jeff this is the owner of the 338 Ultra you bought.
Here in Wisconsin we have 2 spots that may have it .
Midwest Shooters supply had some this week I got 8 lbs. But there on line website does not show any. They have it in the store also Superformance.
Also try Recob's website. as another. Powder valley would be another but they are out.
I would think you try RL-25 or H1000, but hard to get again. My new one I want is RL-33 for the 338 Norma and my 338 Snipe tac
God Bless guys
Jeff