Where can I look up the federal tax ID for a charity? On the IRS website, you can look up the name, but not the actual number. I want to verfiy that the number is legitimate.
I don’t want to use any of those third party sites that come up in the search.

Guide Starhttp://www.guidestar.org/
The organizations Return of Income, Form 990, is a public record, you can IRS to see it. If you have found the organization is a recognized charity, you really have no need to find its employer ID number.
Try the Better business Bureau. But that number is the equivalent of your SSN. Really not public information. The bank that the org. uses has already verified the tax number.
You can try Lexis Nexis.