Check out Primrose Path Nursery. They are located in my State. I had problems with survival. I have very low survival with most of Terra Nova creations. You do not have to buy from them but at least when you see the ones that Primrose Path has created you know they have been tested under the cold, snow, conditions.
http://www.theprimrosepath.com/listings/heuchera1.htm

The other thing you can look for is if they say it is a Villosa. I have good luck with those.

Next your planting conditions. Forget thinking they will survive in clay. Sun for 4 hours morning sun is better. I dig the area and removed all the clay soil down to at least 10". I then I put down pea gravel and on top of that 2" of compost. If you do not have compost use a high quality potting soil with fert. Then the Herc then compost on top. This will give you good soil and drainage.

This what helped me have survivals. I lost so many I was ready to not buy another Herc I did not care how pretty they were.

Here is a link that might be useful: Primrose Path Nursery

My survival rate went to 100% when I stopped treating these as shade plants. Most are in sun until around 2 pm and don't seem at all picky about soil. I really don't have any clay areas however.

Kevin

Ach, I was wondering about the sun issue. I'm trying to find something low growing and hardy with a color other than green (ferns and hostas will be behind it) for a quite shady north facing area next to the garage. Any suggestions?

Hey, I'm just north of you, up by Cambridge. This is my first year with hercs, and I'm crossing my fingers. I found a BEAUTIFUL deep glowing red "Melting Fire" at Menards and planted three of them. They're in part sun with my hostas, next to the garage under a birch tree. I can't promise it'll survive, but it looks like what you're looking for.