I have to agree with what has been said. Most hair canvass is not washable. But (and I don't remember where she got it) my wife bought a bolt of a washable "hair canvass" made of nylon and polyester, woven, not felted like Pellon (tm), for use in the liturgical vestments she was making at the time.

Yes, regular canvass can be used, or any other firm non-stretchable fabric. Of course it needs to be pre-shrunk if you are going to wash the finished garment. Since you have experienced seamstresses in the household, just explain what it is that you are needing the interfacing for.

Sounds like your progress is good!