I have no idea why 3.3.3 is a no-go to that board but yes, you do have options, such as buying a board that as support in the official releases… The beta testing topic is here: Copter-3.4 beta testing start
You can use a GCS with firmware upload capability like, for example, Mission Planner, where you can select the beta firmware. But if you want to download it, the latest RC (in this case rc1) is here: http://firmware.ardupilot.org/Copter/beta/PX4-quad/