I would start with the IRS, it has a search tool for their listing for entities:

https://www.irs.gov/charities-non-profits/search-for-tax-exempt-organizations

Then I would use Guidestar to look for filings:

https://www.guidestar.org/

And if the IRS revoked it, but you have all that info, I would apply for reinstatement:

https://www.irs.gov/charities-non-profits/reinstatement-of-tax-exempt-status-after-automatic-revocat...

Hope that helps.