champion stamp co. at 432 west 54th st in manhattan sells hundreds and hundreds of vintage us stamps for all occasions. they are always in mint condition of course and they have large quantities in stock at all times. check out their catalog on-line or stop by their store (where you can spend hours, as i did, trying to pick out the perfect stamps for your wedding invitations!). you can also place an order via phone or email.
here are some of the pretty botanical themed ones that i picked out for my invitations/rsvps (i will post my invitations later sometime):
some other beautiful ones i came across in their catalog as well: