16 or 18" fry pan - where can I find one?

When you start getting into those sizes, carbon steel is both more common, and makes more sense.

Any reason you've excluded it?

The reason you will not find in typical store is because most home stoves wont hold them . They will require 2 or 3 burners. Also average household quantities of cooking are smaller. A lot of home sinks are to small to hold them. If you must buy one go to website Edward Don corp. Check out their site. They are restaurant and food service only. Volrath, Wearever and some Japanese imports are OK.

You might check places that specialize in church and grange cookware. Those are the most usual places I see large pans like that.

I doubt you'll find one non-stick, though.

Nor, frankly, do I think it necessary. Properly cured carbon steel and cast iron is as non-stick as you're going to get. I'd also be leery of non-stick over an open fire, because the heat might release toxic fumes.

Nearly all of my pots and pans came from restaurant supply stores.  In my area, Houston Tx, we have various stores.  I usually go to Ace Mart.  However, for camping, I like to use cast iron.  You should be able to find a cast iron in plenty big sizes... if it's hot enough, with a little oil in there, you shouldn't have any sticking problems.

Try Sysco.com they have 14 teflon, and yes they do make 16 and 18 inch ironstone coated pan.but only available from rest supply stores.

Here's a high quality non-stick paella pan, they have it sizes up to 19 3/4":

http://www.culinarycookware.com/bourgeat-paella-pan.html

If you absolutely need a handle, their biggest non-stick frying pan is 15 3/4".