If you're not particular. You can use a cutdown .30 cal rifle barrel. I saw a .30 Luger which had been rebarreled with a secton of 1903 Springfield barrel. The owner said that it shot great.
.30 Luger barrels also come up on eBay and the gun auctions fairly regularly.
Here's one with 9 hours left.
http://cgi.ebay.com/Old-NICE-German-...QQcmdZViewItem
Ron