WARNING: dont even try this without having enough dps. im at 30k DPS on my witch doc and im having a hard time bringin the treasure seeker down.
edit: that is on Inferno of course
edit2: you can port to the Crystal Collonades first, grab Tyrael and then go to the Gardens of Hope 1st Tier for some extra DPS
get the quest "Prime Evil" and teleport to "Gardens of Hope 1st Tier"
Treasure Seeker can spawn anywhere in the Area shown on the minimap
http://s1.directupload.net/images/120531/s2tegggv.jpg
There will also be a pack of rares or unique monster like 75% of the time, probably even more often.
treasure Seeker spawned 7 times out of 20 runs. one time he spawned 3 times in a row.
he can spawn anywhere in that area, starting on the steps to the area wich is shown in the screenshot. dont even bother goin further. if hes not there and you killed the rare/unique pack just start another game.
Not the Easist one to farm but if you can manage to bring down the Elites/Uniques and get a little loot of the treasure seeker its definitly worth it !