45/70 is a fairly common cartridge. I'd guess you can find it in most stores, and don't need to special order.
Either way - factory 45/70 loads are sissy rounds. They are meant for older rifles. If you have a modern Marlin or similar, you can load them to much higher than factory. And if you're shooting a lot of 45/70, the reloading costs will pay for itself in short order!