Where does it state that Empress Tianzi's real name is Jiang Lihua? In the Wikipedia and Wikia for Empress Tianzi, it states that her real name is Jiang Lihua.

Where does this information come from? It is never stated as such in R2 so, what side information conveys this?
Empress Tianzi's real name is never stated in the anime series; it's only stated in the Code Geass: Lelouch of the Rebellion R2 official guidebook.
According to pixiv encyclopedia (Japanese),

中華連邦の頂点に君臨する天子の本名である。この本名はTV本編には出ていないが、最終回前に発売されたアニメディアの付録のコードギアスの小冊子にて判明された。 [...]
[...] Tianzi's real name. Even though her real name does not appear in the TV anime, it was stated in Code Geass's appendix booklet that was released before the last episode. [...]

Researching further, based from tare-pannda's blog post (Japanese), her real name was stated in Code Geass Lelouch of the Rebellion R2 The Complete Official Guide Book (Japanese) where the blogger mentioned the content of the book:

DRAMATIS　PERSONAE　～キャラクター解説～
[...]●天子（蒋麗華）[...]
DRAMATIS PERSONAE ~Character Explanation~
[...]・Tianzi (Jiang Lihua) [...]