I also purchased used factory crossbars that came with no hardware. This is what I ended up buying. It’s a really tight fit due to the length of the hardware and the curve/contour of the cx-9s roof.
My eBay link didn’t work. The title of the item is:
(x4pcs) M6 x 25, Button, Torx, Black, Washer, Screw, Shoulder, UTV, 6mm