Ransom brokers appear randomly, like companions and book merchants. But this might also be a bug, if it is then I don't know how to fix it : )
But! Take a look in the tommylaws text tweak compilation, there is a tweak there that lets you sell prisoners to the tavern keepers!
Even better, just follow this link to the tweak I am talking about.
http://forums.taleworlds.com/index.php/topic,46290.msg1235263.html#msg1235263
It is in the second half of the first post, below "Andargor".
There are also some other prison related tweaks, take a look at the compilation, here.
http://forums.taleworlds.com/index.php/topic,61735.0.html