16" and you will have better luck finding one at a HD over Lowes. Plants stores may have them too.It will sit in the water pan so you essentially are still using the water pan but not with water but a saucer..LOL Make sure to foil it to keep it clean. Congrats on the new smoker and pending use!!
Congrats and welcome. Its always good to see more Q'ers from the Bay State! I am near you as we'll in Amesbury....been using my WSM since this past summer so I'm new to it too! Let us know how it comes out...and hey, go Pats!
A good dry rub ( I use Harry Soo's "slap yo daddy" rub which can be purchased on Amazon ) and a temp between 250-300 and some Apple or Hickory wood chunks to start. Try using the 3-2-1 rib method ...works great for me.I'm not sure why some curse the water pan .... It can be a lifesaver if you run too hot and I have NEVER been let down by it. My ribs have a great crust and are very tender and juicy every time. Just try what you think will work for you !!