(Note: I am cross-posting to the perennials forum, so check out the responses over there, too.)

Let's get a list going of the best sources for a wide variety of groundcovers and smaller-scale plants.

I'll start:

Forest Farm Plant Nursery: www.forestfarm.com

Bluestone Perennials: www.bluestoneperennials.com

My urban garden (due to size considerations) is a low-growing groundcover/rock garden, and I am consequently always on the hunt for that totally unique GC to stick into an empty spot. (Groundcover/rock gardens are the perfect gardens for plant collectors, eh?) I am limited to Zone 4b/5a hardiness, which narrows the field of available plants considerably. I am especially interested in sources for cold-hardy plants.