There are a number of brands of steel frying pans 18" to 20.5" if you look under camping cookware. None that I've found are nonstick, but at least one comes preseasoned. This one at Amazon.com is seasoned, not terribly expensive. Fry pan link
The largest nonstick I know of is my 14" that I bought at Sam's Club under the Bakers and Chefs brand. I also have a Simply Calphalon 12" chicken fryer w/lid which is about 4" deep which holds a pretty good load of pasta.