This is reaching back some 10 years or so
(thank God for archiving my e-mails) but excellent local (to you Americans, that is) source is:http://toolclip.com/
I used their model 48-V (although 48 is the exact same thing without a white plastic coating - but coating is good for rust prevention).
They can be used for either 11mm or 1/2" masts - the tighter you screw them in (via the hole on the bottom) the more the jaws close and the tighter the grip. Worked very well for me for years (and still does). As I mentioned, it must be close to 10 years since I last (and first) ordered those - but their website seems to be still on (and still looks the same
) so, I would assume, they're still supplying them...