Pylons in Terraria allow you to set up a teleportation system for simple and fast fast travel around yourworld They can be offered by NPCs, and the NPCs will offer Pylons relative to the biome that they are living in.
You can just buy a Pylon from an NPC who is happy. To make them delighted they need a home, some friendly neighbors, and they will all have various Biomes that they choose to livein As an example, according to Redditor Nick_Smith11, he has actually found that having both the Arms Dealers and the Nurse living in the Ocean biome will result in the Arms Dealer selling you the Ocean pylon.
It should be kept in mind that any NPC can sell the pylon for the biome that they are in, it will just be much easier to make some NPCs better in particular biomes that they like livingin To check which biomes an NPC chooses, you can inspect their Bestiary entry.
Because of the fact that the 1.4 update has just launched, the specific way this all plays out is still a bit strange. Numerous players appear to be reporting various results for which NPCs choose which biomes, so this might either be some general confusion, or it might be world dependant. It is a good idea to inspect your own Bestiary instead of relying on information online at the moment.
Remeber, each NPC in a biome will need their own house, so make sure you build everyone a home to add to their happiness.