Early 20th century warehouse(s) in 15 mm???

[FifteensAway]

Here in USA, we have a couple of companies who produce windows and doors.

I suspect the advent of 3D printing will actually reduce availability rather than increase.  The downside is that in the 3D print world, model railroad stuff is exorbitant in comparison to 'made for miniatures gaming'.  I think that has been long true since model railroading generally requires having an expansive wallet.  We are a 'captive' audience.  Here's the thing, higher prices were partially justified by very small runs of a given product generally - for model railroading.  With 3D printing and generally on-demand printing, that is hoo-ey.   

[Rick]

If you can't find a good source for windows then the least worst option is to put closed shutters/boards over the window area and leave it as that - it's a warehouse, and closed shutters are better security than a window that can be easily opened!