It depends on where you are, to give you that answer.
In the Shopping District of WC, you go past the fountain, to the end of the street. In Krokatopia, it's on the island where the Krokosphinx is. In Marleybone, it's the shop is in the square around the fountain, next to the car. In Mooshu, it's the shop directly behind the Crafting Teacher and Regeant Vendor. In Dragonsphyre, it's the shop directly across the street from the games you do for restoring your life, mana, or potions.
And each one of them have a sign that shows a floating island with a building on it, as well as a bed icon, meaning 'house sold here, furniture sold here'.
The exception to that rule being Grizzlehelm. In Northguard, they only sell the furniture, as no homes are available yet for that world. And you'll find that shop to the left of Bjorn Ironclaws, in the last building before you go through the gate to Ravenscar or Vigrid Roughland. 8)
I forgot to mention there's also the Crown Shop, which is the icon with the crown on it, in the upper left hand corner of your screen. There you can purchase the new school themed homes with crowns or gold coins.