Okay. So while reading your opening post I was thinking you are asking about the actual dipstick gasket, the O-ring under the dipstick cap. But now folks are directing you towards an oil filler tube gasket thread.
Now I have to re-assess. Are you asking about he dipstick cap O-ring or the oil filler tube the dipstick screws down upon?
If it's the dipstick cap gasket it is an O-ring is Lycoming part number 74068. The REAL gasket company makes a red silicon type which seems to last longer. And there isn't anything too special about the O-ring as long as it's petroleum resistant like the MS25913 series O-rings.
If you were in fact asking about the oil filler tube gasket then see the link in the second post in this thread.