Brighton fan here. Off to the home game Monday but will be watching the away match with my TT Sports Boost. You may have to find a local pub who are showing the game. Realise this may prove more difficult the further away you are from Sussex.
Alternatively there is NowTV (possibly a dirty word on these forums) but rather expensive way of watching the game, especially as you're already paying for the service.
Here's to success over both games and Derby in the final!!