Thanks!
I already contacted them. They wrote:
Unfortunately, we do not offer screws for our devices from our shop.
However, we also have an amazing community, with awesome members that are quite active and interested in testing and trying, and sharing new possibilities with our Fairphone.
I welcome you to check out our forum for like-minded people that share your interests and could be using the same product. They might be able to recommend a store to purchase this. The screws we use are Phillips #00 screws (4 mm length).