psychseth

I have a SBE II and have become consistant going 3 for 3 when shooting clays. I would like to get an extension and figure i might as well get the biggest i can right off the bat to avoid any buyers remorse if my learning curve catches up. Does anyone know where to get the tubes that will add 5 more shots to the guns total copasity (sp?) (12 rounds in the gun)? so far i've only found the Tac Star off of cabela's site and it brings the total to 8. Thank you, Seth