There's no such thing as 'lifetime supplies' for reloading components. Even for us geezers. Buying as much as you can afford is not a bad idea though.
Your issue is looking for 7x57 Mauser in bulk. Not a great deal of demand. However, your local gun shop can order whatever you want in whatever quantity you want. Be prepared to pay up front.
Brands will be an issue. Midway, Stateside, while not being an exporter can tell you what's out there and its availability. For example, Winchester 7mm Mauser brass is seasonally made only. Remington appears to be gone, Norma's supply is sporadic and that leaves the pricey Nosler($87.99US per 50. Be more up here.)
Epp's lists the brass, but doesn't give a brand.