I believe they've implied once or twice within the games/anime that while it's not required to start your journey with a region's starters, most beginning trainers either travel to the regions professor to receive a starter or the professor sends them to you himself or via assistant.
In X/Y Prof. Sycamore's lab is all the way in Lumiose city and he sends Tierno to give you your starter. Prof. Juniper sends Bianca out with the starters in Black2/White2 since the starting town in that game(Aspertia City) is literally on the other side of the region from her lab in Nuvema Town.
Bulbapedia has some info on this subject actuallyApparently in the anime, beginner trainers can receive their chosen starter(of the region's three primary starters) at any Pokemon Center.