You can buy prestamped postcards (which are pricey if you're only needing one), or you can just buy the $.37 stamps and put it on a postcard...more expensive, but that's what we ended up doing. HTH!!
Not that I know of - I found them on the US Post Office's website. You should be able to order them through there. We ended up using the regular rate ($.37) stamos for our postcards.