I used the one from the link below back in June that has a QR code and allows you to add it to your Apple Wallet.
It'll be interesting to see if requiring proof of vaccination could be counterproductive. Vaccination cards aren't hard to forge so you might see a bunch of unvaccinated people getting in with fake vaccination cards, especially if they can just show a grainy cell phone photo of it. I guess we'll just have to wait and see how the honor system plays out.