Hi Harry,
At our place we use a lot of organic and fair trade items, and we get most of those food items through UNFI - United Natural Foods. Be aware that their primary client is grocery stores, and their bulk foodservice offering is limited.
I must note that it is my personal opinion is that "ethical and humane treatment of animals" and "meat slaughter" are mutually exclusive concepts. But, as a colleague, I would advise you to look for a Kosher meat supplier - it is my understanding that humane slaughter by hand by a specially trained butcher is part of the Kosher dietary laws, although I'm not sure if humane treatment during the animals life is required as well. Maybe someone else here on the boards can speak to that? Or, perhaps a local synagogue would have some resources for you on sourcing humane meats locally?
However, short of a visit to the farm and the slaughterhouse floor, I'm not sure how you will be able to confirm that the methods meet your approval, and are as "humane" as they are advertised to be. I don't believe the FDA regulates humane slaughter, but maybe there is a watchdog group that does?
Good luck!