19mm 22mm and 24mm grommit pack at halfrauds for £1.49, You have to buy the pack of three.
With concern to removing the arm, apply lots of plus gas/wd40 the night before, give it a slight wiggle as you pull it off and you'll be fine. I've taken one off a 12 year old sax and a 11 year old sax no trouble, you'll be fine