Pete, I'd recommend checking a website called gunbr0ker. They're a site that allows private owners and small shops to sell their products, so they usually have rare and hard to find firearms, accessories and ammo. Just Google gunbr0ker and type in large rifle primers. They have quite a few options but likely on the pricier side. Hope this helps.

MidwayUsa seemed to have a decent selection of primers available.

academy.com/c/outdoors/shooting/ammunition/reloading/reloading-powder-primers multiple brands listed as being in stock

Pete, I would check Outdoor Limited at : https://ammoseek.com/reloading/primers?ikw=large%20rifle