There are only 2 options for fiber optics on the P99.
Walther makes a set of red fiber optics - I think Hal bought a set - U can ask him about them. And, Hi Viz makes ONLY a front fiber optic sight - No rear.
U can buy the red FO sights on ebay - they seem to always have a new set there - as well as the front Hi Viz sight.
If I remember correctly, Hal didn't like the FO sights as much as he thought he would.
If I were gonna get night sights, I'd probably buy Meprolights. But, even though the standard sights are just pop in - I have seen some people claim that they occassionally have to do a little filing to make the mepros fit.
I personally like the stock sights. I shoot at a darker indoor range. And, anything else but white dots are harder for me to see.