Hirst is the only plaster mould company that I am aware of. I got some "building kits" made from these moulds from ebay
UK. Nice stuff if you really need lots of terrain, but plaster is heavy and very fragile. The usual careless gamer will cause many chippings, revealing the quite plaster underneath. In the end I opted not to use these often.
Mordheim alternatives:
1.) Original paper models
2.) Dave's Games paper models (pdf's)
http://www.davesgames.net/ (also a bit fragile but easy to make copies)
3.) World Works Games paper models
http://www.worldworksgames.com/store/
4.) Vatican Enterprises
http://www.wargamescenics.com/ affordable robust realistic 25mm buildings (no ruins)
5.) Kobblestone
http://www.kobblestone.ca/ nice set of city buildings, my basis for a medieval town
6.) Tabletop World
http://www.tabletop-world.com/ nice detailed buildings (Inn ruin!) but realistic scale, not symbolic like most other terrain
7.) Ziterdes
http://www.ziterdes.de/ziterdes/en/produkte/listview.php?catID=64&subcatID= affordable buildings, ruins and the best castle in 28mm, also other terrain