Brandon, 7mm Mauser and 7×57 are the same cartridge just a different name. Be careful though there's also 7.57r Wich is the rimmed version of 7mm Mauser but since it's rimmed will not work in your rifle. So you want to purchase 7mm Mauser or 7×57, just make sure it doesn't have an R at the end since that means it's rimmed and won't work. As for where to purchase ammo at for my harder to find calibers I use a website called ammoseek. Just Google ammoseek and type in the caliber you're looking for and if any websites have any in stock it'll find it. I've already checked and several options to choose from. Hope this helps and you're very welcome.

Brandon, yes, 7mm Mauser and 7x57 are the same thing. I have had very good luck with the S&B 7x57mm Mauser 140gr and 173gr Soft Point available here : https://ammoseek.com/ammo/7mm-mauser