Hello!

You could try the Fidget Stick at Marbles: The Brain Store or Pinch Me Therapy Dough -- It's therapeutic and has a calming effect (https://pinchmedough.com/)

My brother still plays with a Slinky occasionally.

We have fidget cubes. There are buttons to click, balls to roll, disc to spin, switch to flip, dials to turn, and I do not remember the last. They work well for us.

Amazon has some great stuff. Rubiks cube is good too.

Whatvtype of fidget gadget would they like?
My son switches from spinny things to squeezy things or something small to hold and grab.
Have a look on ebay or amazon for ideas as something might catch to eye!