Planar Binding allows a wizard to attempt to bind a celestial, elemental, fey or fiend to one’s service. Only clerics seem to have the spell Conjure Celestial and Druids or Warlocks have Conjure Fey. Wizards appear to only have Summon Lesser Demons, Summon Greater Demon or Conjure Elemental.
Planar binding states:
With this spell, you attempt to bind a celestial, an elemental, a fey, or a fiend to your service. The creature must be within range for the entire casting of the spell. (Typically, the creature is first summoned into the center of an inverted magic circle in order to keep it trapped while this spell is cast.)
If a wizard wants something other than a demon or elemental in their service such as having a high level lawful good Fey or a Celestial like a Ki-rin - what way could they possibly conjure one that could be kept in a magic circle long enough to use Planar Binding?