If you are talking about the Strange Souvenir, this strange item is found when you talk to this backpacker in a random Kalos hotel four times in a row.
Of course, all the hotels are in the following places:
- Ambrette Town
- Couriway Town
- Cyllage City
- Camphrier Town
- Geosenge Town
- Coumarine City
Hope I helped. :)
Source and Experience