o rings - water fill cap

[Cutwater28GG]

so it appears a few of the o rings on my boat are starting to show their age and fail

for instance
the water fill cap o ring.
the o rings for the round hatches in the cockpit to get to the downrigger electronics.

Where do people buy these from?
do people know the specs of these?

 

[knotflying]

take the old one off and try a good hardware store. They usually have an assortment. Or you can measure to old one and look on line.