The appropriate item is called a Medallion of Thoughts, and it allows continual use of the Detect Thoughts spell. Unfortunately, the price is listed at 12,000 GP.
You might want to ask your DM if she can have it anyway. Maybe the DM will say it's okay, or maybe you can get a quirky one that smells like flowers whenever you activate it (limiting its use in certain situations, but not so badly that the item is worthless).