I'm not having any luck finding this, but is it ok to provide financial aid to a potential trade partner in order for them to build a harbor? If so, how much can be given for this?

Thanks ahead of time!

Why wouldn't it be okay? Plenty of people do this in order to get new players into trade rings. Since the harbour costs 200k that is the minimum you'd need to give.

3 million is the standard.
you could include 50 tech from your part if that is the case.
(if you don't have a harbor that is)

It's absolutely fine.

If the guy is in your alliance, I'd say give him three million. If he's not, then one million would be more appropriate considering you will have really no meaningful contact with him after this point. That is more than what a harbor costs anyway, and he could buy some infra to get him to the necessary amount of citizens for another improvement still.