Here are some sources of old west buildings that I’ve used for 25/28mm.
Old Gloryhttp://www.oldglory25s.com/index.php?cat_id=53Old Glory old west buildings are resin and have a very good level of detail. They have lift off roofs, detailed interiors and come with separate windows and doors. They also have a line of customizable old west buildings in which you choose which style front you want to go with each building.
Arnica Real Estatehttp://www.arnicarealestate.com/ Arnica buildings are resin and the walls and roofs all come as separate pieces. They do not come with floors, and only the exteriors are detailed. The level of detail is pretty good and most come with separate doors and sometimes boardwalks.
Sarissa Precisionhttp://www.sarissa-precision.co.uk/ Sarissa has a line of laser cut wooden old west buildings in both 25mm and 28mm. They have pictures on their site showing size comparison with figures. These buildings have lift off roofs and the detail looks good. I haven’t seen these in person yet, but look forward to getting some.
Ertl CowtownErtl has made (still making?) plastic Cowtown playsets which contain a number of buildings that scale well for 25/28mm old west. Sets and individual buildings come up frequently on eBay. I only have the saloon, but judging by that these have a very nice level of detail on the interior and exterior and paint up quite nicely.
O Scale Railroad BuildingsI have a number of O scale railroad buildings which work well with 25/28mm. These can be found frequently on eBay or in stores that carry model railroad products. Examples include train platforms, depots, water towers, ranch houses, etc.
Perry Miniatureshttp://www.perry-miniatures.com/rpb1.htmlPerry miniatures has a nice plastic American farmhouse with a lift off roof. This kit has a very nice level of detail.