You may try contacting The National Christmas Tree Association
I have found a number of sites by seraching "Christmass Tree Growers Association" through MSN and found a lot of listings. I found them while looking for a farm to get my tree from.
You can contact the NCTA at:
1000 Executive Parkway, Suite 220
St. Louis, MO 63141-6372
Phone: 314.205.0944
Fax: 314.576.7989
Email: [email protected]