I know many glass shops and independant installers in AZ. You do not need workers comp. As far as liability insurance, you ideally should make sure they have it, although most will not. However, that being said I would highly suggest using a sub and paying per piece unless you are established and doing a good volume. I don't have a glass shop, I just provide lines of credit for glass shops (lightning capital), provide software (glassbiller) and billing services but if you need installers let me know and I can give you a list!