Very occasionally good banjoes appear on eBay, but this is not a reliable source if you need one NOW.
A year or two ago (?), Packard Woodworks sold a surplus number of a Delta banjoes that would work for that lathe. I doubt they have any left, but a call or e-mail to them wouldn't hurt to see if they have any remaining stock.
There are also some web sites of businesses that cater to power tool restoration (or antique tool restoration) that sell tools or parted-out tools, and you might be able to find a replacement banjo from these on-line resources/vendors. Keep searching on Google or other engines for "wood lathe tool rest holder", or some such set of keywords; "Banjo" might not be the best keyword to rely on for these kind of searches.
You might list a "want ad" request on the WC site, or on the AAW's "Want Ads" page [ http://www.aawforum.org/vbforum/forumdisplay.php?f=3 ] for the banjo that you need.
Good luck finding the second support for your new tool rest. (I managed to find a second banjo for my Jet 1642 on eBay a few years ago, and I use it with my Jamieson hollowing rig's home-made backrest.)