IIRC the 350 Sierra is a Jamison exclusive...It is the one and only bullet suitable for this chamber.
Mystic Precision (Jerry) deals in that brand so he may be able to help you.
If you are thinking about lathe turned bullets you should know there are a few guy experiencing stability issues...Which means a custom tighter twist barrel.
FWIW the 338/408 has readily available bullets and is a better choice in any long range situation. If you need a bigger bullet the 50BMG is a better place to start...When you start factoring in the cost of replacing barrels I doubt the 50 is any more expensive to shoot.
The only place this thing really excels is at 2500+ yards.