Try the Union Square farmer's market. They're sometimes open in the summer on Fridays, but they're year round on Saturdays.
I've dealt with a few farmers there, but really liked these guys: http://www.grazinangusacres.com/ Ideally, if you want something other than burgers or hotdogs (which are really good), you should call them and ask'em to bring what you want (i.e. organs), then when you pick'em up, just tell them your name and they'll get your order from the truck.
They used to show up on Fridays too, which is too bad, because it's a bit of a long commute for me to Union Sq., so I just wind up ordering from northstarbison.com instead.
I first found'em at the Rockerfeller Center farmers' market which is only open July 25-August 31st Wednesday-Friday, near where I work, so it was very convenient, but they haven't been there in a while, and for some reason last year there were no meat vendors there, so had to take the subway down to Union Sq.