You will need to call Umarex but they may not have them in stock at this time -- I think the cost is less than $100 shipped. You can also buy a Mac1 match barrel and have "Rich from Mich" machine it for you. Heck, Roald may have one laying around as well and I'm sure he can chime in to clarify this or not. The Umarex contact info is at the top of page 1 in the Information forum.
http://www.network54.com/Forum/583224/
By the way, if looking at Mac1 for a barrel - don't take the prices on their web site as gospel. I ordered one and found the web price quite different from my credit card billing. Also, Rich was the one that machined it for me and did an excellent job.