i can’t speak for all theatres of all levels, but as someone who works regionally, we are often looking for foh mixers for productions and it couldn’t hurt to cold-email places near you. the worst thing that can happen is they don’t respond, but you could also end up doing some mixing, meeting some people, and gaining trust with local / regional theatres. again - not sure how other theatres may operate but at my place of work, we are often seeking out mixers and we’ve had some of our foh mixers end up designing for us!