Dave Mitton wrote:

I'm looking at putting a ground cover on a 30 ~40 ft section of a slope about
10" wide. I'm tending toward using phlox subulata or stolonifera.

By some rough guesses, I'm looking about over a hundred plants. Depending on
how quickly I want them to fill in.

Where would I be able to buy something like that in the Greater Boston area at a
reasonable price??

Dave.
Sylvan Nursery in Westport, MA is a massive wholesale/retail outlet.
http://www.sylvannursery.com