Techimpex. There's a few suppliers online, including the manufacturer itself. Part prices are very high, so you might want to consider taking your cap to the gas hob fixerman in your town and have him sell you one of the same size and then drill and countersink a hole in it.
I overhauled ours two years ago and bought two new burner caps and the stainless top girder, which amounted to about €200 overall (we were in Italy so bought from Techimpex via the local chandler). Make sure you protect the screws from corrosion with some high temperature grease so they come out again in the future.
