I want to purchase the black iris. How do I go about finding someone willy to sell?

NGA does not sell plants although there are some members who do sell plants.

There is a forum for Irises. There is a sticky at the top of the Irises Forum page with a list of recommended vendors.
The thread "Vendor list" in Irises forum

You can also look at our Green Pages (listed under Tools & Apps) to find a list of vendors that sell Iris.
https://garden.org/greenpages/...

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.