Remington made over 60,000 rolling block rifles for Egypt in the 1870's. The cartridge is the 11.43x50R Egyptian, also called the .43 Egyptian. It is a bottle neck case, similar to other cartridges of the same era like the 11.15x58R Spanish (.43 Spanish), the 10.75x58R Russian (.42 Russian or .42 Berdan), and the 11.15x60R Mauser (.43 Mauser). While similar in appearance, none of these is interchangeable with any of the others.
I know of no source of ammunition, but many of those rifles have been converted to .45-70, or to rimmed pistol cartridges like the .44 Magnum.
Jim