Conway,
Bill Crews usually has the largest selection of Skymasters (
www.skymaster.com), Owen Bell can build one for you custom, and I usually see some on Ebay.
In any case, Owen can do a pre-buy and set your plane up with O-2 windows, cargo pod (with a camera hole if you want), winglets, gross weight increase, wing tanks for 160 gallons in standard wings, wing extentions with 20 gallons per side, underwing tanks (100 gallons total), or just plain ferry tanks. If you want FADEC IO550s, or floats on the thing, I'm sure that can be arranged too.
Randy Hurt