I just attended the International Umpires Camp in Atlanta last weekend. It was hosted by Tony Thompson, assigner for multiple college conferences in the south. I knew that before I attended. What I was surprised to find out is that Tony also is the assigner or significant member/President of The Multi-County Umpires Association which handles most of the youth ball and high school ball in the greater Atlanta area. A good number of Multi-County members were at the camp and they were good umpires. An impressive group. There is also the local college association, Southern Collegiate Umpires Association. Tony assigns local college non-conference games through that association. Another excellent group of umpires. Many do and will soon be calling D1 games for Tony and the conferences he manages.
Tony has a website for his camps that contains his e-mail and phone contact information.
http://www.baseballumpirecamps.com
I hope this helps some.
I am sure there are other associations but these two seem to have what I would deem as better umpires, dedicated to the profession/avocation.