Direct link for those too lazy to search. With the refcode stripped off. :)

https://www.amazon.com/VIO-Vive-Foam-Pad-1pc-set/dp/B01MDUIG39/

(edit) Looks like that one's sold out, here's a link to the mesh version:

https://www.amazon.com/VIO-Vive-Foam-Pad-Mesh-1pc/dp/B01MRE7HD0