Just type in the name of your game, then "evolution x trainer"
So a search for Halo 2 trainers would be "Halo 2 evolution x trainer"
You could also try a search for "game" "xored trainer"
I can't think of any big sites, but I have found a few sites that offer packs of trainers. Also, you can find trainers in some of the underworld sites that are looked down upon by X-S and I would get suspended for posting a link here.