What is the standard finder's fee paid to someone (who is not
incorporated but very well connected) by the firm contracted to plan
and produce a corporate holiday party for 400 in New York City?

Hello,

I have some experience in organising events for companies, the fee for
organising an event would be 15% of the total budget (food, location
hire, animation,...) If they need help finding something, they pay 15%
of their share to that person.

If you have any more questions or comments, feel free to ask, I'll see
what I can do.