Rifles and ammunition of that caliber fall under the category of destructive devices in the US, so export for civilian use is not going to happen. Overseas, most of the 14.5 that exists is for the machine guns, and is either armor piercing, incendiary, or HE incendiary, all of which would be controlled for import. As well, most of that ammunition is a touch more powerful than the ammo that was meant for the PTRD and PTRS, so you likely shouldn't use it anyway.
We seem to have enough trouble just getting .303 these days, which has many consumers here in Canada. I doubt that any importer could make a profit bringing in ammunition for a rare piece like the PTRD. Heck, most guys here in Canada whine about $6 a round 50 cal (when you can even find it at that price). At $50 a round for 14.5, they would sew their wallets shut.