I bought some of these (eBay 150799123186, the vendor is "piercove1") recently. They're the "bullseye" type, 14mm in diameter, and the price, less than US$1.50 each, delivered, was attractive to me.
I find this a useful size, sufficiently small to be mounted (in my case with cyanoacrylate adhesive (read "crazy glue)) on the top of the camera body without occupying excessive space. They are a nice fit for an Ansco 5x7 or a Korona 4x5.
I have no connection with the vendor and my only knowledge of him is that he ships quickly.