I seem to get a lot of requests this time of year from wedding planners. Old postcards make great "Save the Date" cards for weddings. For smaller wedding you might be able to get away with sending original cards, but a more economical approach is to reproduce the cards or incorporate the images into a new design.
Here's the end result of easy wedding invitation project.
I offer high-resolution scans of most cards for a nominal fee, which can easily be incorporated into Save the Date cards, or any type of graphic design project.
There are tons of print shops that will do low-volume postcard printing - VistaPrint, printsmadeeasy.com and postcards.com are a few - but not all do matte finish, which is usually better for that "vintage look". Some printers can do different paper stock which can emulate the original linen cards.