Not sure where you are located. I am in NW sub too. Chalet in Wilmette is one of biggest nurseries I have ever visited and typically carry a wide varieties of of heuchera and other plants that other nurseries do not have (typically mature size as well). I was there last weekend. They do have some dark purples including Obsidian, Midnight Rose at that time. You can call them and ask them to check before your visit. Very nice and friendly staff.

3132 Lake Avenue Wilmette, IL 60091
(847) 256-0561
www.chaletnursery.com

Alternatively, you can order from http://ilovehostas.net/, an online nursery in WI. Very good service. I ordered 2 Cajun Fire from them last Sept, in 3" pots. The grew to almost mature size this spring. Last Sunday night I just placed orders for 3 Bella Notte, 2 Paris, 1 Fire Alarm as I can not find them locally. They emailed me Wed that it has been shipped and should receive them tomorrow.

http://ilovehostas.net/coralbells.html

I got all my Obsidians at Home Depot in August. They do carry them and Midnight roses. But not all the time.

Best of luck,
Vivian