You could use something like http://www.digikey.com/product-detail/en/FP30-200/237-1021-ND/242467. Connect the 2 secondaries in series across the 24V supply (I've chosen a 30V transformer because your supply is more than 24V unloaded). Leave the primary open circuit. Then you will get 15V across one of the secondaries, which you can rectify and feed to a 7805.
Or build your own switching regulator, as I suggested in a previous post.