To purchase back copies online, visit the San Francisco Chronicle Store. The store has a 60-day archive. Copies of articles older than 60 days, but printed since January 1, 1995 are available in the archives on SFGATE. SFChronicle.com has an archive of content going back to its inception in March of 2013.
You can purchase single digital copies of the paper through PressReader.com. Those archives go back about two months. These copies are a print replica, similar to the e-Edition and can be downloaded as a PDF. Additionally, the replica e-Edition (available to subscribers) has a 30-day archive.