The battle frontier is in Hoenn after you beat the Elite Four. Board the boat in Slateport there and you'll gain access to it. If you go Right and Up for a bit you'll find this building.
http://prntscr.com/motmff
Inside, I think the girl in the middle will teach your Pokemon moves they can only learn from special events and stuff like that in exchange for BP.
http://prntscr.com/motmzt
I forgot whether Extremespeed on Dragonite is 10000 BP or 20000 BP but yeah that's another way to get it.
suigin got a reaction from k4far in "You can get a Special tutor to get Dragonite with Extremespeed too using BP in the B