I love vintage ephemera. There is something about the craftsmanship involved in old printmaking that makes me swoon. So naturally, I adore the idea of using old postcards for save the dates.
Ideally you want to use blank cards that are specific to the region you plan to marry. However, not all of us are lucky enough to fall under that umbrella, so for the rest, there are beautiful generic images that will work just fine. Here are examples of some of my faves:
Vintage inspired save the dates from Oh So Beautiful Paper.
Destination wedding postcard via Style Me Pretty.
Via Get Married.
Assorted Niagara Falls postcards via Vintage Glam.
If truly vintage isn't practical, how about printing your own? Flickr has a large community of members (like Suzie Que) who post images from vintage postcards (and other types of ephemera) which may be reprinted for personal use. So eco friendly (no envelopes needed!) and fun. Cold weather is just around the corner so easy peasy diy projects are a good thing to get lined up.