When I first started collecting heucheras years ago, I ordered from Eminence Meadows. Their plants are healthy but not overly large. They carry some rare varieties that I haven't been able to find elsewhere. ( Link below)
More recently, I've ordered from Plant Delights in NC and Bluestone Perennials in PA. They carry a lot more varieties than they used to!

Here is a link that might be useful: Eminence Meadows

I buy almost all of mine from Garden Harvest Supply and sometimes from ilovehostas.net You have to search heucheras and heucherellas seperately at GHS.

Here is a link that might be useful: GHS