Interesting Ki Prestige classes?

[golentan]

Are there any? Looking for a class which could compliment a ninja especially, but I'm just working on a build at the moment, considering my options, trying to come up with a fun character to really ENJOY, you know?

[Psyren]

Evangelist will advance your ki pool, ninja tricks, sneak attack and even has 6+Int skills. The spell-like abilities your deity gives you tend to be Cha-based as well. There's a guide to the PrC floating around out there but I can't find it for the life of me.

[grarrrg]

Technically, there are no Ki Prestiges for Ninja.
And there are barely any for Monk for that matter.

Evangelist, as Psyren pointed out, will work, but isn't necessarily better than Ninja (isn't necessarily worse either).
If you don't mind taking at least 3 levels of Monk and 1 level of Paladin you can sneak into Champion of Irori (http://www.d20pfsrd.com/classes/prestige-classes/other-paizo/c-d/champion-of-the-enlightened). That grants a fully stacking Ki Pool.
There's also, sort of Brother of the Seal (http://www.d20pfsrd.com/classes/prestige-classes/other-paizo/a-b/brother-of-the-seal). You'll need to dip Monk, but only 1 level this time. It doesn't actually give or stack with a Ki pool, but it does let you use Ki in combination with Awesome Blow.

There are a variety of other classes (or archetypes) that grant (and therefore stack with) Ninja Ki pool.
Monk is the obvious one. It doesn't get a Ki Pool until level 4 (at least one archetype gets it at level 3).
Shigenjo (http://www.d20pfsrd.com/races/other-races/featured-races/arg-tengu/shigenjo-oracle-tengu) Oracle also gets a Ki Pool, but not until level 7.

[Psyren]

Another option is Magus (or VMC Magus, if you prefer), which grants an arcane pool, which you can then turn into a ki pool via Ki Arcana (http://www.d20pfsrd.com/classes/base-classes/magus/magus-arcana/paizo---magus-arcana/ki-arcana-ex).