Where can I get a roll cage built?

You can start by looking at the Map of North American Cage Builders. Odds are good there's not going to be one in your town. That's ok, it's common to have a rally shop a couple hundred miles away make your cage.

It's also very likely there are cage builders much closer to you that build roll cages for the circle track racing or road racing in your area. Most of them will never have built a rally cage before. That doesn't mean they can't, or that they need to get certified before they do yours, it just means there's going to be more hand holding on your part. They can bend tubes and weld them, that's not the issue, but the design, style, and philosophy of an optimized rally cage will not be known by them.