As with a lot of wildcats I shoot (most of them) The easiest way to find ammo(to shoot it)----- is to make it yourself-----from ----.270 brass,anneal case,trim to 2.5,F/L size in Newton die (C&H,tool and die)trim to length,chamfer,and load----
Suggested load, 140gr bullet---57.0gr 4831------minus 10% for start--watch for pressure signs
If you don't want to go through the minor hassle of case forming and would prefer proper headstamped brass, Jamison makes 256 Newton brass. I'm sure you're tracking on this, but make sure to use the 264 caliber bullets and not 257 like many people have been known to do. I got an Encore barrel really cheap because a guy said it just wouldn't shoot. When the barrel and reloading supplies showed up in the mail the package had a bunch of 25 caliber bullets in it, explaining the inaccuracy. It's a great cartridge. I used 6.5-06 data to work up loads.
Some of the smaller gun shows also have a few old/unusual ammo guys. Kokomo is April 11, Tipton and Holt Road are April 18. I remember Holt and Tipton having some of the old/unusual ammo guys but Kokomo would be my last place to look, they are more 7.62X39, 5.56, 9mm type dealers.