There are some good free/open courses online for linguistics, such as some offered by MIT. They won't get you a degree, but you can still learn the content and get some sort of certificate (for a fee) for completing them. If nothing else, that's a way to start. Might even prepare you for grad school in linguistics, if you can otherwise get in based on your other experiences.
I don't know of any linguistics BA programs (and if there are any there would be very few), but there are some MA-level (or certificate-based) ESL teaching programs. So depending on your career goals this might work for you, and it's possible (but may not likely?) they would have some more theoretically oriented linguistics classes you could take in the program.
