Formally speaking, they can be ordered from Cobra (who makes them themselves) via a Starboard dealer. But availability has not been that high, I hear. And my experience with national distributors tell me that it will not exactly be a stock item (though I think it would be a good idea to stock a few since they are not expensive).
I would try your local dealer and see if they can contact the distributor. You might be able to persuade them to ship them quickly (fedex?) if you're prepared to pay some extra.
I have two set's on order myself, but no sign of them yet. If you're desperate and nothing else works, I might sell you a set.