i download custom airplanes from http://www.flightsim.com you will want to go there after peak hours otherwise you won't be able to log in. They have the biggest selection of custom airplanes as well as other stuff for flight sim.
If you are looking for addons, ie......Aircraft, scenery, etc.....try www.simviation.com That site has addons for all the flight sims. Also you can go to www.sim-outhouse.com or www.avhistory.com. All these sites have lots of good stuff. Check out the links sections at these sites too.