There isn't one!
TextEdit, Mail, Stickies, and all the rest reimplement it for themselves
each time, seemingly. I spent a good five minutes or so trying to prove myself wrong by
finding a standard interface to the Find functionality we see in so many Apple applications.
A reason for the "Find" submenu of the standard Edit menu in Nib files. But no.
Can this be true, or am I just a complete idiot today?