Here are the various scales that border onto 15mm and a discussion of their suitability as 'alternative' scales for buildings and scenery.
1/144 (11.2mm) - Roughly equivalent of N-Scale railroad models. This is a popular scale for die-cast metal Aircraft and some action figures. However vehicles in this scale look very undersized next to 15mm models. Buildings also look far too small, particularly domestic buildings. In FoW aircraft are represented in this scale.
15mm (1/107) - The FoW website describes the game as being "15mm (1:100) Scale". My understanding of scale is that 15mm and 1/100th are different sizes (see below) but I suspect the models are 1/100th to the top of the head with eye level at 15mm, hence the dual scale description. There are plenty of buildings for 15mm wargaming from a wide range of manufacturers.
1/100 (16.1mm) - The true scale of FoW miniatures. Some plastic kits are available in this scale. Modern military vehicles also come in this scale, in both metal and plastic. Alternatives to Battlefront minis in this scale include Peter Pig which are also 1/100th. Buildings at this scale are usually described as 15mm although architectural models are also available.
1/87 (18.5mm) - Otherwise known as HO scale, this is a popular model railroad scale. Buildings can vary a little in proportion so some manufacturers 'under size' their buildings. Consequently HO can be used as an alternative to 15mm. I've been told that this works best with 'industrial' buildings but that houses etc. look a little too big next to 15mm figures. I touched on the issue of scale in this blog some time ago and found that this is one subject guaranteed to put fire in the blood of any gamer/modeler. The above descriptions and suggestions are based on feedback I got from The Miniatures Page message boards (Here and Here) and various other forums online, as well as my own experience of size comparisons.