If you can't find the correct part for the SYE you got used, I'd go with one from Advance Adapters. You obviously know how great their support is since they helped you even though you don't own one of their products, so you can see why many have chosen to use AA SYEs on their rigs. I know it means shelling out more cash, but in the long run you'll be much happier than moving back to the slip-yoke.
Another option is talking to the person you bought it from; if he told you he knew they sold the part separately, he was either lying so you would buy it or he has contacts that you don't. Either way I'd be very unhappy and I would most definitely be talking to him.