Ruger is still listing the 7x57 in M77 Mark II Standard Walnut and Synthetic. Also in the No. 1 RSI and A, at least it is still listed on their website.
If Ruger does discontinue their offering, Dakota or other semi custom shops will continue to offer the round. Many of the European gunmakers will continue producing it too. I haven't checked, but would bet CZ has it as part of their standard line.
I would imagine any competent rifle builder could build you one.
It is a grand old cartridge. Properly loaded, it will approach 280 Remington ballistics and surpass the 7mm-08. I'm sure a search on GunBroker, GunsAmerica, or any of the on-line gun houses would uncover a new or lightly used 7x57 for a decent price also.