This is for the U.S. only-
Ironmind (
www.ironmind.com) use to sell ring weights, but I don't see them listed, write and ask. They do sell a ring handle and some guys use that, plus a link and loading pin to make a similar thing-
http://ironmind-store.com/R-Ring153-Handle/productinfo/1258/ .
Sorinex makes a Hammer trainer that some folks like-
.
Bert does the Highland games stuff too, so they might carry other things too-
www.sorinex.com .
There's the North American Scottish Games Association that is a lot of help-
http://www.nasgaweb.com/getstart.asp .
And a few other things-
Steve Pulcinella, who's a member here, was/is a high level Highland guy. He runs Ironsport Gym,
www.Ironsport.com and does stuff for
www.elitefts.com . Maybe he'd know the inside scoop.
Also-
Some guys at
www.powerandbulk.com and
www.irongarmx.net are active in the sport, so they might help you.
If in Europe, I don't know, but maybe try
www.pullum-sports.co.uk , they've been in the power sports for years and might know.