How much do you want to spend?
You can always make stuff yourself which is a lot of fun and generally cheaper, though it tends to take longer.
You may want to look into stuff on model rail road sites as well as the more traditional fantasy type stuff, depending on what you're looking for.
4Ground and Impudent Mortal both make nice
MDF stuff, though Impudent mortal is more sci fi maybe or at least more modern.
http://www.customkingdoms.com/ makes some awesome looking stuff, but
tbh it's a bit expensive and already painted
Hirst arts molds for DIY stuff are really awesome! Granted there's a learning curve for the casting process but it's a lot like legos: you can build almost anything. And the site has plans and instructions for different buildings so if you like one of them, you can recreate it yourself. Just need the molds. Which is an investment but a worthwhile one
imo.