These FOSs are on auction at eBay, Auction Arms and GunBroker classifieds from time to time. Run a search and something will eventually come up.
However, the front sight will only work(?), or fit, the receiver on the 3.4 (short) P22 version. The 5" has a worm (set) screw behind the front sight, which secures the bbl sleeve to the fake comp, that will be blocked by the FOS due to its protruding length.
EDIT: Here is a link to one of these Hi-Viz P99/P22 front sights: