Hello, I am looking to get a harmonica to play mary janes last dance. I know it is in the key of "G" but does anyone know if its a diatonic or chromatic?

I'm assuming that we're in the same key, the verse chords being Am-G-D. I use a C harmonica and it works out pretty well.
However, if you try to play to the record you're going to find that the original is detuned slightly. My guess is that they sped up the recording after it was finished. Not enough to move a full key though.
If you play the song in regular tuning, the C harmonica should sound fine. If not, you'll have to keep trying different harmonicas until you get the sound you need.
Hope that helps you.

The one Scott slid my way some years back was a G with an impression in it which gives it a different sound; maybe it was a busted one....