It may seem counter-intuitive, but while there certainly was a quietly thriving fandom pre-movie, it wasn't particularly old. I'm a Steve/Tony shipper rather than an Avengers-gen reader, but that was considered the juggernaut ship even back then. I'd say I read pretty much everything there was before the MCU started, and it was almost all written after 2000. Steve/Tony fic really only took off with Civil War, even though there'd always been jokes about it prior.

If you trawl through Tales of Suspense, and the two LJ comms (http://stevelovestony.livejournal.com and http://cap_ironman.livejournal.com) that's about it. For other pairings try Marvel Slash.