Can anyone tell me where I can find a replacement cartridge for my old foremost shower which they don't make anymore. I don't want to have to tear it out.

I'd start by looking at and calling the larger plumbing supply houses...

If it is old enough to be discontinued it won't have a cartridge. It will have compression type stems, those can usually be rebuilt with washers and "O" rings and seats.

Foremost - Vintage Faucets