You have to have an industrial grade sewing machine before you even think about sewing seatbelts. I'm not sure you can even buy webbing as a consumer since there are laws that shops have to be certified to be able to sew them for customers.
I also Have this problem,especially with ssnakeoyl. I called them for a dark red webbing, which I have been unable to find. They wanted $10 a yard. My locale supplier gets $1.50 a yard a big difference,but hasn't beenable to supply me with the right color. Now I know everybody is going to tell me it is certified material or some other BS. You can buy complete after market belt sets cheaper than material from ssnakeoyl.
Sorry about the soapbox.