Well, a great item is the headband of ninjitsu. But it's more expensive than 6000 gp. I don't really have a good recommendation for something in that price range.
Do you already have:
Magical weapon(s)
Magical armor
Belt of Strength or Dex (depending on build)
Amulet of Natural Armor
Ring of Protection
Cloak of Resistance
If you can combine the cloak with the Elven Cloak for better stealth, that helps.
I also like Boots of the Soft Step, to hide from tremorsense when moving slow. Combines well with the feats Dampen Presence and Hellcat Stealth for maximum sneaky.
You might find Spider climbing slippers useful too, or some other way to scale walls.
A few portions of invisibility, darkvision, and gaseous form can come in very handy. Alter self (or the kitsune feat that lets you being a fox) could be handy for squeezing through places or escaping restraints.
A Pathfinder Pouch is a nifty, small bag of holding that you can turn on & off and it doesn't detect as magic. I have an assassin that uses one as her emergency escape bag (extra lockpicks, some daggers, a few potions/scrolls, etc.)