Right now, we don't offer our RSVP cards as a stand alone item in our assortment. RSVP cards are only available bundled with your invitations order.
You can adjust the number of RSVP cards you need right from your draft, though - from the RSVP tab, click "RSVP Quantity" to change the number of cards you need.
Working from the app?
Once you've started your draft, make sure to include RSVP cards. You can edit the cards in the draft flow, and will be able to edit the quantity on the Review Your Order page.