Are you talking about the HS1 hat? They had them in the normal saddlery stalls last year.... they come up the same size as normal skull caps but are a bit bulkier to look at.... Hubby has one and its much lighter to wear....
a friend of mine bought hers from fieldhouse i think. they're fantastic - worth every penny...when i went to olympia theyre were none anywhere - and derby house, robinsons etc are yet to stock them...good luck!
I think I bought mine from the people that sell the 'Style' XC boots and Rodney Powell body protectors, sorry can't remember their name but they're always at Badminton....good luck!!