I have some Pikestuff HO scale building that have windows but no clear glass. Question is what do I use for the glass ie: material name and thickness and where do you find it?
I have and use various thicknesses of plastic, which I have simply saved through the years from free sources such as blister packaged items. This has worked well, many, many times.
You can use any clear acetate or plastic material, like the stuff used in a lot of everyday packaging from the store.
Small windows can also be done by stretching a bubble of white glue across the opening. It doesn't necessarily produce perfectly smooth 'glass' like plate glass. And on a humid day, if the air conditioning isn't on, it can turn white again, though that's temporary. I tend not to do that with buildings. But it makes good auto glass, because it conforms well to the shape of the opening. It makes pretty good curved glass.
I use a thin clear sheet styrene/plastic from Evergreen. There is a liquid product that comes in a bottle and you spread it between the window panes but I've not tried it. Some master modelers use microscope slide glass cut to size but I'm never going to try that!
That liquid product is white glue. Though it's thinned out a little with water, which is a good idea.
All good ideas, I have used them all with success. Another thing you can use is crystal clear scotch tape, this works pretty good in building windows.