Rubber Bung-thing for Brew Pail

Greetings!
I am making my own 3 gallon brew pail, and I'm having a bit of trouble. I can't seem to find the generic black rubber bung-o-ring thingy that you see on every brew pail to jam your carbon release into. I've checked around at local hardware stores but since I don't really know what it's called, asking for a " black, bung-o-ring thingy" hasn't helped me find it. Does anyone know where I can find them? Who manufactures them? Where can I get them in small orders? Thanks!

Can you use an o ring that will fit your airlock snuggly and fit it in?

It is called a grommet: http://www.northernbrewer.com/shop/grommet-for-lid.html.

Thanks, that answers my questions!