There is a DM precedent that spending 1000 gp will obtain followers as the Leadership feat. Followers cannot be used in combat.

Though the pdf is the ultimate houserules reference, the ban list is horrifically out of date. Thus, a copy is hosted here. Please, please, please cite your sources. ¹⁾

Hopefully, the DMs will also deign to elaborate on their reasoning/lore, for our own amusement and elucidation.

Though previously banned as “setting-specific material”, Magic of Incarnum is now accessible as a +1.

Ever since Brutal Mike defeated his evil uncle at the Peak of Pain and reopened the Well of Incarnum, incarnum can be used by player characters. The obscure country of Negana has a tradition of Incarnum, but little is known about it.

Though once okayed by Voice of Dyno, the alignment-based paladin variants are banned.

Per Word of Drew, > Paladin of Freedom is also not allowed, but there is a Holy Liberator Prestige Class in Complete Divine that is functionally the same thing.

The rationale is that the Blackguard and Holy Liberator PrCs are “closer to core” and also cover the thematic niche of an evil or Chaotic Good Paladin.
There are also the Pious Templar PrC from Complete Divine and the Divine Champion from the Forgotten Realms Campaign Setting which are alignment-agnostic Paladin-like PrCs.

There are also theories that the older image of paladins as Strict Lawful Good is to be upheld, as no other option is available at low level.