Someone asked me the same question a little while ago on an M-400 Starduster thread here on WWDX. To help our colleague out, I did a little looking through a few years worth of receipts and found that the vendor I bought it from was Eagle1.com. My receipt refers to the hub as the "Workman 6 hub." The problem is that the business doesn't appear to still be around. If you can find one, the hub is threaded to screw right onto your plumbing pipe.
I've looked on E-bay and several dedicated amateur radio sites hoping to find more of these, but I haven't had any luck. If you check with some of our colleagues on the M-400 Starduster thread, maybe someone there had some success.
Good luck.