The US Postal Service is selling Batman stamps, and since they're forever stamps, if you bought enough you could use them forever, or at least until you run out of stamps or life, whichever happens first.
You can purchase them in sheets of 20. I ended up getting 5 of those sheets, but since I plan to actually use them to mail stuff, I also got a framed set to keep, pictured below after unboxing. There's also other stuff you can buy from the site.
Update: And of course, right after I publish this, Gizmodo has a story about the USPS having been hacked. Although it looks like it happened and was discovered a couple months ago and they'd been working to fix it. So hopefully my purchase of these stamps didn't make me a victim, particularly since it looks like they went after employees more than customers.