You can't find it anywhere. There is no such thing as a certificate of electronic filing. Whoever is asking your for it doesn't know what they're talking about. Ask them to show you an example of what they're looking for.
If they show you a Form 8879, or tell you that that's what they want, explain to them that Form 8879 is only used when you pay a professional tax preparer to e-file your tax return for you. It is not used when you file your tax return yourself with TurboTax. Form 8879 is called "IRS e-file Signature Authorization."
Maybe they will accept the "electronic postmark" that TurboTax creates, or your e-file history. Either of those shows that you e-filed your tax return. Or ask them if they will accept a Tax Account Transcript or a Tax Return Transcript from the IRS. A Tax Account Transcript shows some basic information, without all the detail of everything that's on your tax return. You can get either type of transcript at the following link on the IRS web site.