Simplicity 2159, goes to a size 22.
Here is another pattern that goes up to what they call grande size or a 72" hip.
http://www.adiradance.com/dance_costume_cp/PT-803/BD_PAT/i3/o/Adjustable+Ghawazee+Coat+and+Turkish+Vest%2C+Pattern+%239803.html
http://isabelladangelo.webs.com/turkishcoat.htm
It's very easy to design your own.
Only if you already know how to draft patterns...
Not really. You know your own measurements, right? That's half the battle right there. I know a lady who was taught a very similar pattern as the first thing she sewed. She still uses that pattern for everything because it *is* so simple and yet can yield some stunning results.
Most people fear pattern making because they fear themselves messing up. Just use muslin for the first try and see how easy it really is.