If you need to get them by application, A Harley dealer. They are the same as a Sportster.
Since they are a fairly universal O-ring, you can get them from an injector service shop too.
Twin Motorcycles sells Buell stuff in the Netherlands.
MARK: several UK dealerships including including warr's....sykes....newmarket....norwich show them in stock. but don't waste your time or money tracking them down as they show retail on EACH @ $9.40 US. use the supplied info below and purchase locally from any quality auto parts or fuel injection service shop or O-ring and seal supply house. you'll get 4 or more at same price of 1 at dealership. You're welcome. Merry Christmas!
early 2004 and up all XB top and bottom O-ring the same.
must be viton....durometer rating should be in the range of 65-90 plus.
Inside diameter is 7.9 mm
Outside diameter is 14.9 mm
O-ring section width is 3.5mm