Another way to find potential sellers is to scroll through the iris database, with a search for 'iris black' which will bring up anything with black in the name. If you like the looks of a particular iris, open the database entry, then click on the notation 'X members have this plant' which will bring up a list of folks who actually have it in their garden. Some will have a notation 'will trade' or something along those lines. You can then send a private email asking if they might be willing to sell (or offer to trade for something you might have). Please do note that it is inappropriate to approach folks who don't indicate they are interested in trading/selling. Good luck with your hunt.
