As per Ann's reply, take measurements of ID of spar, and OD needed. Also you need to define the length of the plug
Here is link that shows the use of wooden plugs on a Boston Whaler: http://duckworksmagazine.com/15/howto/bamboo/#.XUJXpOgzaUk
If you ask around your local area, there are always people with wood turning equipment
and skills whom are happy to build objects.
Additional a plug can be 3D printed, once again by local 3D printing hobbyists or professional services. They only need the for mentioned dimensions too, to generate a CAD file for 3D printing generation. I can generate the CAD file for a 3D print if that helps.