Ah, old school. (if they're not too big)

You've discovered the reason that ALL cycling shorts used to end just above the knee. (And if the fabric was too stretchy, they'd ride up anyway.)

I doubt most places will take cycling shorts back once they've been worn. Try selling them here on TE maybe.

Or perhaps you can find some silicone gripper material that can be applied liquid? Wouldn't hurt to browse the fabric stores.