Treasure map VIII
The next map can be found beside the body of a hunter, east of Winterhold. The man has been killed by Horkers living by the coast. The exact location of the body can be seen in the screen below.
Read the map and head to Dragon Bridge - if you haven't been there and therefore can't use fast travel, keep heading west of Solitude and eventually you will reach it. Right before the bridge, between three trees, there's a bag with a key to the treasure chest. You will have to dive to reach it, as it's at the bottom of the lake east of the bridge.
Of course if you don't want to look for the bag, you can ignore it completely and open the chest with picklocks. The lock is however very difficult (master level), so you will probably have to spend some time with it.
Treasure map IX
A bit to the west of the Abandoned Prison, right beside the waterfall, there used to be Lucky Lorenz's Shack (not marked on the map). Currently it's all but ruins, destroyed by a falling tree. The owner can be found dead in the water. Search his body to find the treasure map.
The map shows the Riften bridge and a waterfall, behind which the treasure should be. The location can be found right beside Broken Helm Hollow, behind a wall of water.
Treasure map X
The last map can be found at the Stony Creek Cave, at the very east of the region, south-west from Windhelm. Enter the cave and read the alchemist working for the Bandits. In his inventory you will find the items you're looking for.
The map points to Lady Stone, found on a small island in the middle of the lake south-west of Whiterun. The treasure chest can be found underwater, at the very bottom, somewhere between the Stone and Ilinalta's Deep.