Bring a completed invitation to the post office to have it weighed; many require postage for at least two ounces, currently 61 cents. Have a reply card and its envelope weighed as well, to ensure that you don't over or underpay for that postage.
Ask what's available at your local post office, or browse through a wider variety at the U.S. Postal Service website: http://www.usps.com/. Vintage stamps can be purchased from philatelic societies. They are worth the amount printed on them - but they can cost much more, since they are collectible and limited in quantity.
Take your invitations to the post office and request that they be hand-canceled. Machines print bar codes on the envelopes, but hand-canceling- just marking each stamp- keeps invitations neat and prevents damage that machines could cause. (Maybe you'll have better luck with this than I did-my post office refused to hand-cancel for me.)