I need a gas cap for a 1988 Hurricane I/O 4.3 - any suggestions where I might purchase one?

Welcome to the Owners Forum!

There are quite a few places you can purchase a gas cap for your boat.  It also depends on what type you have.

You can always try your Hurricane dealer, or a number of on-line marine retailers like:

iBoats:  http://www.iboats.com/

Overtons: http://www.overtons.com/

Defender:  http://www.defender.com/

West Marine: http://www.westmarine.com/

If, due to the age of the boat, you have an oddball cap, you can also try Great Lakes Skipper:

http://greatlakesskipper.com/