You can't buy any Link Stones, even not in Reborn City, but there's a Link Stone available after you're locked out of Reborn City.
Ametrine Mountain B3F, start by reaching Ametrine F1 by climbing up the Waterfall in Celestine F1 (Celestine Cascade entrance) or jumping the ledge and climbing down the Waterfall in Ametrine F2 (Calcenon/Route 3 and Route 2 entrance), open the ice-Strength puzzle by pushing the boulder from one side of the Waterfall and going through the path from the other, go through the Dive spot and through the cave opening at the right (surf over it downwards), resurface to B1F and go to the left in the ice skating puzzle, go into the Dive spot and through the cave opening at the low left into B2F (the Lapras location), go through another cave opening at the bottom left to B3F and find one hidden on the lower left rock. (From the In-Game Item Guide v2)
Also, to make spoilers, it's