You can find a lot of these on Amazon where they go for about $20 a piece. However, I've found a website from Russia that sells them substantially cheaper. I've got got an order from them with no problem (they accept Paypal), they took about a month to arrive.
They come in flat packs and each kit contains several dual-side-printed sheets that are perforated.
You simply punch them out and they are slotted so slide, more or less easily together, usually without the need of glue. Most of the buildings took only about 15 minutes or so to put together.
Most building are designed to have a "flap" side that give you access to the building interior.
They come in a number of different scales but they have a range of medieval town buildings that are roughly true 25 mm in scale. They would also be useful for old-fashioned buildings in Central Europe up through World War II at least. Here's some Perry miniature figures to give you an idea of how they compare to 28mm. I'm not a stickler for exact scale so these work pretty well for me.
I've gotten some of the HO scale buildings to see it they are usable with 28mm (which I use exclusively. They are considerable smaller than the medieval range but might work well with 15mm.
The building on the left is a 1/87 HO scale Russian church, the building on the right is part of the medieval range. However, I found this 1/87 church that I think will be useful for a small chapel or mausoleum - it will probably become part of the Frostgrave ruins.
So, if you are looking for a very quick, not too expensive way to create a colorful medieval town, Clever Paper could be useful.