This is a list of sites that I found that sell Impatiens. If you would like your site listed please let me know. Some may have one or two but a few on here have quite a few to choose from. I don't sell plants yet but, I am thinking about it. I would like to get a count of people who would be interested if I did.
In the US:
note Crug is not a mailorder nursery
Here are some other sites that I believe will be of interest.
Ohio State University
Has a collection of Impatiens and is keep Germplasm in its priority genera.