Where are the NPC traders in ORAS, and which Pokemon do they offer?
Since it's probably gonna take a longer while to make a full list, I'm making this answer into a community wiki, so anyone can update it as they find more traders.
- Rustboro City: Slakoth -> Makuhita ♂
- Brave, Holding X Attack
- Fortree City: Spinda -> Skitty ♀
- Naughty, Holding Magost Berry
- Pacifidlog Town: Bellossom -> Corsola ♀
- Calm, Holding Heart Scale