You might try booking through Volaris. They are still showing codeshare connections with Frontier flights.
Just out of curiosity, what is your use case for booking a F9/Y4 codehare flight? The fares were not very competitive from what I saw and the only time I ever used it was to book a Y4 flight via Concur coming back from a work trip (the Y4 flight wasn't listed under their code in Concur, my company's standard for booking work travel). It ended up being a pain since I couldn't checkin online and an agent in GDL had to call someone to manually fix my reservation. Volaris is a neat niche airline and we don't see much discussion about them here.