are you talking about the thing Pete Frampton uses, they usually only cost like a $100 or so i beileive the products name is "Heil Talk" or something close to that if they don't have it in any guitar stores(probally cause it would be kinda pushing it if a person used it all the time) ask them if they can order it for you, most guitar stores i know will do that. i think the notes that you sing take on the sound the guitar makes, i had a freind who had one i didn't try it out cause that's a personal item like a jock strap or something. ok

peace out,

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Jake Sommers