There are at least two traditional witloof farmer's unions here in Belgium. One of the requirements is that the farmer saves his own OP seeds.
We happen to live near one of those farmers, and in the season, we often buy his witloof. It's sweeter than any other witloof we know, and Rudy (the farmer) told me that taste/sweetness is one of the features he selects for. There's no other place where you can get his witloof seeds.