Sim outhouse at
http://www.sim-outhouse.com/
Is a pretty good place to start. You should be able to find links to the bigger mods like Mediterranean Air War from the boards there. I think they also host a number of smaller mods - aircraft and terrain textures; that sort of thing. Personally if I were you, I would see if Windingman's terrain mod is still available. He is the guy behind the OFF mod but the terrain overhaul is for the regular game. It's very high quality.