I do quite a bit of the specialty type of plantings, it's part of what I advertise. I also do a lot of mail order for my plants. Many of my ground covers come from
Classy Groundcovers - evergreen flowering ground covers, grasses, vines, ivy, .... (Good prices, and free shipping, and absolutely beautiful plants. We've agreed on our fondness for heucheras, and those come almost exclusively from Terranova. Yeah you have to buy a flat, but it's only one and after looking through their plant pictures, you'll play hell ordering only one item. They also let you divide it and get two 1/2 flats.
I'm fairly fortunate so far in my larger shrub and tree suppliers. If I were back east I'd probably look harder at a new place I just found,
Dave's Nursery - A Propagator of exotic plants of all types operating out of New Jersey.. He says he offers wholesale as well as retail, and specializes in unusual.
I spend a lot of time searching for the specialty plants. I hit multiple nurseries every week, which I know is not a good use of time, but it's the only way I can find what I want to install. And I have found that my wholesaler isn't saving me a ton of money until I get into shrubs and trees.