My jungle, corruption, and hallowed biomes all spawned in the same place
By making the Dryad NPC move in, during Hardmode, you can buy Hallowed Seeds from her for 20 silver to create the Hallow biome.
If when you get enough to make a biome and already have the Steampunker NPC, you can make him move to a house inside the Hallow and buy Clentaminator and Blue Solution to spread it faster.