The trainers for those are the members of the Culvier Clan, they're in the following locations:
East Gearen City
Goldenwood Cave
Route 6
Zone Zero
The presumed last member is currently on a pilgrimage
I'd highly recommend trying out Pokémon Reborn if you are in search of a new challenge.
The Virtual League can be participated when talking to specific NPC's starting in the Grand Hall up until West Gearen.
The Goomy Kingdom can be entered through a dive spot in the northern part of Honec Woods if I recall correctly.
I hope I could help at least a bit
After surfing around on tumblr I got heavily inspired by chrisjalejo's interpretation of the Gym Leaders up to Ciel so I asked a artist friend of mine if they'd be interested in an reinterpretation of the missing 5 Gym Leaders.
I must say that the results are quite eyecatching.
My online name is Hyena2305, I'll be ready in ~2 Minutes.
Don't need something special in return, but preferably take Dark Types if available.
I'll be waiting
To get Aron you'll need the TMX for Rock Smash and access to the greater Railnet which is after the 6th Gym
I could give you an Aron, if you don't want to wait that long