Welcome to the Forum!
First off, there is no frame on an MGB. It's a unibody car. And there are several vendors that can help. You should probably start with Moss Motors, Victoria British, and The Roadster Factory. You can access them online with a Google search.
Also, check with your local British car club. If you're near Nashville, the Nashville British Car Club is a good resource. (I'm in Clarksville, and a member. I have a '73 MGB as well.)
Mickey Richaud