Where can I find info about Signature cards?

I'd love to get my hands on signature versions of some of my all-time favorite cards, like Reno, C'Thun, or Yogg, but I'm not exactly sure of the specifics. I know that they can't be crafted, so is the best way just to open the golden caverns of time packs? If I do that, what are the chances of getting a signature? Thanks!