There are two components which make up the internal construction of a kilt. They are the stabilizer and the interfacing. The stabilizer is a small strip usually of broadcloth that goes between the two back buckles. The interfacing is the large strip which spans the entire kilt from the left edge of the under-apron all the way to the right edge of the outer-apron. The straps are anchored to the interfacing. It is the interfacing that is normally, today, made from hair canvas.

In the old military kilts that I have been re-conditioning for the Canadian Scottish Regiment both the stabilizer and the interfacing are made from a medium weight cotton canvas. This looks sort of like a cotton gunny sack fabric. It is actually sailcloth. This stuff did its job for almost 60 years just fine.

If you cannot find hair canvas there is no reason you could not make the stabilizer and interfacing both from cotton broadcloth.

The thing you MUST look for is a fabric that does not stretch. Not at all. If you grab the fabric and pull, it should not have any give.

In a pinch you could use an old sheet or pillowcase. I'll warn you though, sheeting is hard stuff to sew through.

Almost every fabric store will carry cotton broadcloth. The price is usually minimal. Many have prints on them. This would allow you to add a personal touch to your kilts. This would also be the same stuff you would use for your liner so if you can find it, get extra.