Shimano haven't been making them for a while - so any old stock will gradually be starting to pick up in price.
Stronglight Zicral rings are still available from the distributor in Denmark - and I can see they have stock on the way next month so it looks as if they are still being made. The standard alloy rings are no longer being imported though so can't give you the heads up on them.
But otherwise - Middleburn. They are about the same price as Stronglight Zicrals.