I'm guessing you mean 12mm thick glass? If so that's not a common size glass tubing I see, I've never seen anything thicker then 9mm on glass tubing. Besides custom order glass tubes and condensing 9mm to aprox 12mm I don't think it will be very cheap if possible. Good luck
Dude I wonder how much a simple 12mm thick 14in straight tube would cost about to ask dwb once I get some money hahaha