Q: how do i find a receipt for an app purchased on iTunes store?

See your purchase history in the iTunes Store on a Mac or PC - Apple Support

You should have received an emailed receipt for it, if it's not still in your inbox check the spam folder on the account.

Or you can view the account's purchase history via your computer's iTunes, that will give you details about it (e.g. price, order number) : See your purchase history in the iTunes Store on a Mac or PC - Apple Support