ok, so here is the skinny
no sight blades available anywhere that i know will fit
no complete rear sight assembly available anywhere (99,101 or 66 and 44 revolvers)
beretta adjustable rear sight is not interchangeable
a 92 fixed sight MIGHT fit but i am not sure if the front sight heights are the same on the 92 and the 99... if not its not an easy adjustment since the front sight post is integral to the slide.
if the 92 sight does fit, and it is the right height, there will still be a groove infront of it in the slide.
my idea, if you dont want your local gunsmith to install a set of novaks would be to try the 92 fixed sight, and since you still have most of the adjustable sight, if the fixed works for you, have the smith modify your old sight so the groove is filled with the original piece.... kinda like frankensitghts....
i have a pt 92 that i installed pacmayr adjustable rear sights on.... this was before the 99 was available.. in hindsight, easier to find the pacmayrs now than the originals from a 99