The items that boost Psychic type moves only are:
- Twisted Spoon, found in Malie City Outer Cape or held by wild Abra (5% chance).
It boosts the power of Psychic type moves by 20%.
- Odd Incense, found in Konikoni City Incense stall.
Like Twisted Spoon, it boosts the power of Psychic type moves by 20%.
- Psychic Gem, which boosts the power of such moves by 50%. But, it is one use and not available in USUM.
- Mind Plate, found in Route 1.
Like the other two, it also boosts by 20%.
These are the item that boost only Psychic type moves. Since Mewtwo has a lot of respectable coverage from tutor and TMs, you can opt out for item that boost all types, such as Life Orb, Choice Specs, etc. or even Wise Glasses.
Hope this helps!