You need to create the affidavit yourself and get it notarized from yes, an attorney. They usually have templates for all such legal documents.
Definition- A standard form of affidavit used to help verify whether a potential prize winner of a sweepstakes, contest, or other promotion is eligible to win a prize under the promotion's official rules. The affidavit also includes a publicity release and a release of liability for the conduct of the promotion.
For tax, if you are a tax-payer, you will have your tax-payers registration.and relevant forms.