There are several options: Meprolights, Trijicon, and the ones from Earl's Repair Service. Novak may have something by this point, but I'm not familiar with it.
Mepros are the least expensive and some people like them but, in my experience, they generally don't last as long as the ones from Trijicon. You're looking at 6 to 8 years vs. 12. Also Mepros have some Quality Control issues where they occasionally vary in their dimensions. The result is that they don't fit like they should. I believe someone here had that problem and when they contacted the company, they were told to shave the slide. -Wrong answer. The gun is worth $500, the sights are about $50. If I have to grind on one, the cheaper part is going to have the encounter with the Dremel tool.
Earl's Repair Service uses the Tritium inserts made by Trijicon in their sights as well as Trijicon's front sight blade, so when comparing the two, you're really just comparing the rear sight's shape. It's a personal preference type of thing. Earl's are more expensive.
The ones from Trijicon will probably be the best balance between price and performance. I went with the ones from Earl because I preferred the sloping cut Earl does on the rear sight to prevent snagging on clothes. It's a minor thing and if you have some extra coin, go for it; but it's not necessary.
I like my meprolights for the money. I had some fit problems but eventually filed the bottom of front sight and was it was fine. The company was nice as they sent me two replacements after I bottomed out and broke the screws in the sight.
I just had the Mepros put on a P99 this week. While it is true that the rear sight did need some minor fitting (and yes, the little doco said to file the slide) it came out nice. No problems at all with the front. The warranty slip in there also called out 12 years on the True Glow (which these are.) I took the path of least resistence and had a local shop install them once I saw the rear fit. 15 minute wait and $15 later, I'm glowing in the dark.
I would prefer a Novak-style sight, but he/they don't offer one for the P99 that I know of.
Two sets of Meps on my P99s with no problems. I like the dots on the Meps better than the Trijicons. Had a set of Trijicons on a G19 and the dots just seemed smaller and the white not as well defined for day shooting. Both are good products IMO.
Yes, I like meps better too. And, I actually planned to get a set soon. But if you have to smith the sights a bit to make them fit, I won't bother. I like that the P99 allows the user to easily swop sights. Sights made for the gun should fall into this description I think.