Are you talking about scratchbuilding or buying buildings? What era/genre/scale? There are dozens of good terrain websites out there. For 25/28mm, the hirst arts stuff is a pretty good for nearly any era if you want to building your buildings a bit like lego. Without more info, I cannot really help you more.
depends on scale and era:
If you need for 20mm minis or similar try the railroad model making. Some companies offer very nicely detailed interiors and accessories for buildings (companies like Kibri etc)
If you need for 28mm minis, there are quite few items available:
Try: ex-Ainsty (now crow models), frontline figures, blackcat bases and even eureka who have some interior details (modern with PC etc) also pardulon from germany has some fantasy windows, doors and victorian era accessories!
Hope that I helped you