Not sure they sell just the globe, and probably cheaper to just upgrade/replace with an LED. I swapped mine for this one. Says "wake tower" but works for any boat under 40 feet. Super bright...
Welcome to the forum! Like @mattie said -- replace it with a modern 2-mile or 3-mile LED fixture. It's one of the best safety investments you can make. Also, you won't be replacing bulbs every other year.
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https://www.amazon.com/gp/product/B00K8DBIUK
Boats up to 40ft.
Should be easy retrofit. A lot less batt power reqd.