I find it quite odd that we don't have a terrain thread for this game where terrain can be so interactive and pivotal. So in the interest of the community as a whole, let's start posting links and pictures and guides! And maybe, just maybe. This can become a sticky.
http://www.4ground.co.uk/
Lost of rustic buildings that can fit well into the Victorian era feel or the more Medieval feel. Many buildings you can go inside with models!
http://www.worldworksgames.com/store/
Lots of papercraft terrain of all sorts.
http://www.davesgames.net/
More good papercraft goodies.
http://lonetreegames.com/
Would be great if it were not for the marked out tiles. May work well for the IKRPG.
http://www.hfminis.co.uk/shop?category=accessories~resin-terrain
Lots of nice
misc stuff to put round the battlefield.
http://www.antenocitisworkshop.com/wargames-scenery-terrain/fantasy-scenery-terrain.html?limit=36
A couple odds and ends to add to your own buildings.
http://www.fenrisgames.com/shop.html#!/~/category/id=0&inview=category4298015&offset=0&sort=nameAsc
More odds and ends.
http://ainstycastings.co.uk/index.php
Lots of good stuff stashed in here. Especially ships to make harbors.
http://www.zuzzy.com/
Gaming mats and a few odds and ends.
http://www.hirstarts.com/
Cast your own stuff!
http://www.sarissa-precision.co.uk/
Some good buildings under Dutch/Belgian section that could work for Warmahordes
http://www.bplaser.com/
They have a few tall buildings for steampunk.
http://store.gcmshop.com/
Wild west buildings that wouldn't look too out of place with some paint.
http://www.daemonscape.com/index.html
Some earthworks stuff tucked in there that would work well in Warmachine.
http://pegasushobbies.net/catalog/Peg.-Gaming-Accessories/c2/index.html?osCsid=te05rj6alhfb3j9h2p9utd7n64
Some gothicy stuff that works for Menoth.