Clever Models. They sell craftsman card stock model railroad kits that you can download and print. They offer a wide variety of models in O (1:48), S (1:64), HO (1:87), and N (1:160) scale all done with photo-realism. The variety surprised me. They have buildings usable for Western gaming (I love the small train station!), pulp gaming, WWII gaming, and more. Nice industrial buildings as well. Prices are very inexpensive IMO.
Though made in model railroad scales, shrinking them to gaming scales should be no problem. The owner even mentions doing this at one point. I'd get the O scale kits for 25mm gaming. You can shrink them as you see fit or use them as is for a slightly larger building. Same goes for 1:72 gaming. The HO buildings will work fine as-is with the new Blue Moon 15mm (aka 18mm) figures, or shrink them a bit for regular 15mm. You get the point.
Below are just a few buildings I like: