I’m not familiar with a casting company that lets you use your own metal. That said, you may be able to find a local jeweler or individual jeweler that will cast for you and let you use your own metal.
In the past, I used to cast a couple of local jeweler’s waxes and let them use their own metal. They didn’t want to ship their waxes out of town and appreciated one-on-one kind of custom service.
These days there’s also a local jewelry store who occasionally casts my student’s work using the student’s metal when needed.
I’m not sure of the volume of casting that you’re talking about but you might be lucky if you ask around your local area. Best of luck!!
Jeff