You don't need a site. Scan any legit boarding pass, or save a PDF of one after you check in online. Then change the date with any graphics program. (You can change more than that, but it's easiest if you start with a BP for a flight that will also leave on the desired day from the same terminal of the same airport.) It will almost certainly get you past security, but not onto a plane. The bar code, etc., wouldn't check out at the gate.

However, on domestic US flights they don't check ID again when you board. Once past security with a fake BP, you can board with a ticket in someone else's name. (If that ticket carries a gender-specific given name, it's best to be of the appropriate gender. Gate agents are busy, but may notice this if something catches their attention.)

Added in edit: You don't even need a fake boarding pass. Buy a refundable ticket in your own name, get a BP for it, go through security, get on the flight with the bargain ticket in someone else's name, and refund the ticket you used to get to the gate.