I've been looking for one also, as of yet they are still very few available and the price is, well, 1/2 price of the PS90, Here is the cheapest site I could find one on.
U could probably get 1 cheaper by buying a new PS90, and selling the gun w/ no sight, and just throw a cheap optic rail on it (or, if you already have a white ring site from your current PS90 - put that on it and sell the gun unfired).
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