Without counting Pokemon League and Stadions.
Straiton City ex-gym leaders
Castelia - Royal Unova
Join Avenue - the Fitness counter there
Depending at what level you are:
Route 23 - Audino training with Lucky Egg
Nimbasa - Little Court & Big Stadium, once a day and you get a ton of cash + some items too.
Plasma frigate (near P2 lab) - Colress, once a day.
Colress at P2 Laboratory with lvl 77-80 Pokemon.
In Castelia City with the two game freaks with lvl 74-78 Pokemon. Or the royal unova with lvl 65-68 Pokemon. You need 1000 pokedollars to get on.
In nimbasa city with the big and small courts with lvl 60-65 Pokemon.
Your rival in Driftveil city with lvl 66-70 Pokemon.
Audinos with lucky egg.
Striaton Gym with two on two battles with cilan, cress, and chili with lvl 65-68 Pokemon.
N at victory road with lvl 70-80 Pokemon.
Join avenue with the deli person and fitness trainer. Make sure you have a lot of money though.
Cynthia at Undella Bay with lvl 74-80 Pokemon.
The shadow triad at Iccirus City to to the right of the Pokemon center over the bridges. You will battle all three of them with lvl 74-78 Pokemon.