Check with you local "cute rate" tire shops... Leave them your name and number. Many folks buy new once their mudders hit 50% or so and loose their "new & cool" affect. If the tire shop has your info handy and knows your a potential buyer, they'll give you a call (granted the previous owner doesn't keep them). This is something that upper scale tire dealers won't take the time to put forth the effort for, but the small business will.
Keep in contact with them, stop in every now and then, they'll eventually put a face to the name and a bell will ring when a set of good used mudders are sitting out back.. Trust me, it works, I've picked up plenty of quality used mudders this way for pennies on the dollar. Last time was a set of 35" Micky Thompson baja Claws, 50%+ tread for $100 (all four!!)...