Klaus, If you find a place to buy those in the EU at a reasonable price let me know. I didn't find anyone with the aluminum version and the rare sellers with some carbon version in stock are selling them for extortionate prices.
I'm surprised nobody makes a conversion kit, 4 balls and two spreaders, for a discount.
And be careful, some sellers only have the plastic version (apparently it is distributed by Hobie Cat?) and those are known to fail.
There is a commercial Murrays distributor in France (Select what you want in the catalog, drop an email to email@example.com and he will quote you a euro price), I've never used him, but as murrays seem to have those parts in stock it may be the cheapest way to get it here. Ordering direct from Murray or catsailor unfortunately cost too much in shipping.