Sorry to be pitching stuff here, but I'd rather this go to someone on this forum than an anonymous person on ebay.
I have a lightly used Santana rear rack (made by Old Man Mountain). Not sure if they're still selling them; it will fit on a tandem with V-brakes (or V-brake bosses). I paid >$80 for the thing. First $30 + shipping takes it. (Pick up in Newport Beach at no charge.)
All parts are included except: 4 allen bolts needed to mount the rack to the bike; washers for those allen bolts, and two spacers that go on the allen bolts that attach to the V-brake bosses. If you have a V-brake on the back, I believe you will just need the other two allen bolts. Hope all that made sense.