We just love the notion of using mix-and-match vintage stamps on wedding invitations. While it can take patience to amass enough 15- and 3-cent stamps to match today's postage rates, we find the look to be worth the effort (wouldn't this be just the perfect task to delegate to some well-meaning family member who wants to help with wedding preparations? We think so!).
And of course we have to mention Martha Stewart Weddings, who have long championed the idea - trust them to compose each envelope perfectly...!
Where to buy? Ebay, of course. We're partial to seller SeaJay Stamp and Coin, who allows you to "buy it now" (no waiting around for auctions to end!) and offers topical sets featuring different themes, e.g. flowers, birds, butterflies, etc. Just be sure to look out for "mint" (e.g. unused) stamps.