I'm not really good with names, but I do believe I finished that mission yesterday. Somewhere around the north-western part of Armadillo. Can't be very specific, I tend to abruptly break my route if I see something in the distance. I was riding in the area, seeing a white building and head for it. Then the game said I was approaching a know gang hideout. Then I rode towards it a bit more and it popped up a yellow dot and prompted me to go to the farmer.
I did not accept anything to do with this beforehand, nor did hear anything about this (as far as I can remember). Very cool mission though. So far the most western feeling one I have done.
Make sure you have some dead eye left near the end of this mission.
Found my first new pistol there, dropped by one of the gang members.