1biggun directed me to the "pistol barrel" page at Green Mountain, where they had a 15" .500 bore barrel for $79.95. Twist is 1:18, close enough to the recommended 1:19 for the Beowulf. I gave them the credit card information and it'll be on the way tomorrow for $87 with shipping.
Impactguns still seems to have the best price on Beowulf brass, but their order page is either hosed, or doesn't work with Konqueror. I'll have to call them and order the brass.
I was going to order a .223 AK magazine, but when I went to the closet and dug out an M-16 mag, I found out it appeared to accept and feed 5.45x39 just fine. I'm now unsure what the difference between a 5.45 and .223 AK mag would be, unless the x39 case is a bit more tapered. The Eyball-O-Meter couldn't see any difference with the single .223 cartridge I have on hand.
I also found out that the standard ex-military mag I have accepts 7.62x39 just fine, too. The M-16 mag tries to center the cartridge (more or less) as opposed to the left/right feed from the AK mag.
There's only a couple of dollars' difference between a 5.45 and .223 mag, so I'll probably just go for a .223... the Beowulf case is darned near straight, only .010" taper, so an ideal mag would be almost straight. Since Alexander Arms says a standard AR-15 mag will work, I'm guessing it's close enough.