well I had a hard time finding one and ended up getting two at Napa( Sierra Marine) years ago....
OMC made some but they are always out of stock... I bought two this year on ebay, one came with a teleflex gauge. what MODEL is your motor. my mid 90s V4 looper uses a 1/8th" pipe thread( blind hole) in the heads... I put one in each and have a separate gauge for each as well..
bob