You used to be able to find them on faction comm and they'd expire after a few minutes. Then a lot of the communities who used to publish them shut down, and for the most part you have to find them as they release on Google+ or Decode Ingress. Decode Ingress gives you the tools and hints to decode them but makes you do the actual decoding, which means codes stay more valid longer as fewer people cut-and-paste them blindly into the Intel site.