Spirals aren't great to stretch with, as they increase at an uneven rate (the outside more than the middle), and carved ones are often not as nicely finished on the inside either.
Plus, I had spirals at 16mm, and they were INCREDIBLY clunky! I couldn't wear them for more than a few hours... And they stuck out at stupid angles too XD
Taping (ptfe/bondage) is a great way to stretch, and super cheap. Just make you lube properly, and tape on single/no flare plugs
