Other than the options listed under the find familiar spell in the PHB, and the Pact of the Chain warlock, what other official options are available for familiars that can be summoned by find familiar?
For example, on page 221 of Tomb of Annihilation, we see:
With the DM's permission, the find familiar spell can summon a flying monkey.
I know that several monsters have a "Variant: Familiar" option (which refers to actual creatures of that kind that form a contract to serve as an NPC spellcaster's familiar). This is not about those, but about creatures that offer an option to work with the Find Familiar spell.