Sight glass in coolant tank leaking

Coolant is slowly running out at the base of the sight glass on my overflow tank. I noticed that the sight glass screws into the tank. Making sure it was tight didn't stop the leak.
Has anyone had to replace the sight glass? If so, is there an o-ring used at the base or is it a small gasket? Also, are either one of these available at auto parts stores? Thanks in advance.