Boston Valve Needed

[Icemanxxxv]

I lost my Boston Valve Cap off my Creek Company U Boat 2000. Are they a standard Boston Valve? I need one and was wondering if my local marine store or other online source before I pay Creek company their inflated price. I have seen the valves cheeper just not sure if they fit. Is ther a standard for them or are they sized at the will of the producer if the boat?
Thankws
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