Well if you already have Petilil. Here are the rest:
Gigalith Lv. 35- Trade an Emolga for his Gigalith. (Trader is in house on Route 7)
Tangrowth Lv. 45- Trade a Mantine for his Tangrowth. (Trader is in house in Humilau City)
Rotom Lv. 60- Trade a Ditto for her Rotom. (Trader is in camper on route 15)
Ambipom Lv. 40 & Alakazam Lv. 40- Trade an Excadrill for her Ambipom. And later you can trade a Hippowdon for her Alakazam. (Trader is in building in Accumula Town)
Nimbasa City Various Lv. 50s- She will request any Pokemon for the one she offers. Here is the semi-diffucult method:
To get these trades is rather convuluted. You first need to find the Dropped Item in Nimbasa City. From there, you will be called when you enter random areas by the owner of the item. After the 10th call, the person will call you and tell you to meet in Nimbasa to return their Item. When you do, they will be added to your Xtransceiver. Call them to total 30 times, but you can only call from places which fulfill the conditions. On the 30th call they'll tell you they want to meet at Nimbasa City where they'll ride the Ferris Wheel with you. Call them to total 50 times, in the 40th call they'll be 'SOUND ONLY', in the 50th call they'll be wearing their work clothes, and cut the line in a second or so. Call them a 51st time. wait a day and then call them again and they'll meet you in Nimbasa City and offer to trade, willing to have any Pokémon. This can be repeated each day to go through all of their Pokémon. The Pokémon you receive varies based on your gender.
The areas are as follows:
Behind the Pokemon Centre in Nimbasa City
Near the Officer in Route 16
Behind the fence on the bottom of Route 5
Somewhere near the slope to Clay Tunnel in Driftveil City
Across the first bridge and near the ledges of Route 6
Around the stairs near the Breeder in Route 7
On the Airstrip of Mistralton City
Route 9 near the Hidden Grotto
Lentimas Town near the stairs.
Undella Town between 2 rocks
On the beach in Route 13 near the start from Undella Town which is behind the ledge
Once again. Random Pokemon and a little difficult to do.
The info was all from Serebii.