It was in Legends and Lore in the Egyptian deity section.
Edit: Found it.
"Priests of Horus are, like the god’s avatar himself, paladins. Unlike the traditional characters of this class as described in the Player’s Handbook, they are only of chaotic good alignment. Thus, they are not bound by the normal restriction against chaotic behavior by paladins. Further, they cast spells as priests (not as paladins), but only gain the ability to turn undead upon reaching fifth level.
Requirements: AB as paladin; AL cg; WP any; AR a; SP all, combat, guardian, healing’, necromantic, protection, sun; PW see above; TU special."
Nothing there exempting them from a holy avenger's LG requirement. It only exempts them of restrictions on acting chaotic.