Can I substitute cornstarch for commercially prepared clear glaze?
Unfortunately no. Cornstarch will cook up white while tapioca starch cooks up remarkably clear.
You may be able to find tapioca starch at your local Asian market or if your grocery store has a decent organic or gluten-free section.
That said, if you speifically need to avoid tapioca for some reason, you could try arrowroot powder. I haven't seen it myself, but I've read that it also cooks up clear, but maybe not as thick as tapioca.