Well they still make ammo for the 356 winchester and 307 and they were both shortlived and chambered in very few firearms and not many of them.
The 450 was chambered in a lot more firearms--Marlins, Brownings, T/C Encores, BFR pistols, H&R singleshot rifles, etc.
Just lay in a supply of ammo and it won't matter. And/or you can get reloading dies/brass, etc. for it and you won't have a problem with keeping ammo for it.
I have done the same for my 338 and it was only produced for 2-3 years and only in a Marlin.
458 bullets will never be a problem for it with the 45-70 around.
I think the 444 may be in this same boat also but it was made for a longer time but only in the Marlin rifles until they came out with Encore chamberings, H&R rifles, BFR revolvers, etc. Remington and Hornady were the only ones making ammo for it.