Well, AO3 has only been around since '07/08 I think, but I guess many writers have x-posted their fic on to there after originally posting elsewhere (LJ, FF.net etc).
So your best bet is searching via tags, sorting by Date Posted then going to the last page and work your way back from oldest to newest?
So on Fanfiction.net - http://www.fanfiction.net/comic/Avengers/3/0/0/1/0/0/0/0/0/29/0/ (that's just the Avengers tag under Comics, but you could look through the individual character tags as well here http://www.fanfiction.net/comic/).
And there are a bunch of tags on AO3 you could look through that aren't specifically movie verse, again just sorting Date Posted and jump to last page.
The Avengers (Marvel) - All Media Types (http://archiveofourown.org/tags/The%20Avengers%20(Marvel)%20-%20All%20Media%20Types/works?commit=Sort+and+Filter&page=967&utf8=%E2%9C%93&work_search[complete]=0&work_search[language_id]=&work_search[other_tag_names]=&work_search[query]=&work_search[sort_column]=revised_at), Marvel 616 (http://archiveofourown.org/tags/Marvel%20616/works?page=85), The Avengers - Ambiguous Fandom (http://archiveofourown.org/tags/The%20Avengers%20-%20Ambiguous%20Fandom/works?page=1006), Avengers (comics) (http://archiveofourown.org/tags/Avengers%20(Comics)/works?page=38).
I don't know if you're expecting to find fanfic written in the 60's or something, but with the internet not becoming widely available until the 90's, plus comic books originally being a vastly male orientated hobby? I don't think there's gonna be too many 'old school' fics pre-dating when the MCU originally started in 2008 (though there may be a few based on the original movies/TV shows).
I know some girls who are huge comic book fans, but 9 out of 10 fans I know got into reading comics post MCU creation, and having crushes on RDJ etc, so pretty sure most of the fic out there is going to be film verse or comic book verse written more recently than you're hoping to find ¯\_(ツ)_/¯
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So your best bet is searching via tags, sorting by Date Posted then going to the last page and work your way back from oldest to newest?
So on Fanfiction.net - http://www.fanfiction.net/comic/Avengers/3/0/0/1/0/0/0/0/0/29/0/ (that's just the Avengers tag under Comics, but you could look through the individual character tags as well here http://www.fanfiction.net/comic/).
And there are a bunch of tags on AO3 you could look through that aren't specifically movie verse, again just sorting Date Posted and jump to last page.
The Avengers (Marvel) - All Media Types (http://archiveofourown.org/tags/The%20Avengers%20(Marvel)%20-%20All%20Media%20Types/works?commit=Sort+and+Filter&page=967&utf8=%E2%9C%93&work_search[complete]=0&work_search[language_id]=&work_search[other_tag_names]=&work_search[query]=&work_search[sort_column]=revised_at), Marvel 616 (http://archiveofourown.org/tags/Marvel%20616/works?page=85), The Avengers - Ambiguous Fandom (http://archiveofourown.org/tags/The%20Avengers%20-%20Ambiguous%20Fandom/works?page=1006), Avengers (comics) (http://archiveofourown.org/tags/Avengers%20(Comics)/works?page=38).
I don't know if you're expecting to find fanfic written in the 60's or something, but with the internet not becoming widely available until the 90's, plus comic books originally being a vastly male orientated hobby? I don't think there's gonna be too many 'old school' fics pre-dating when the MCU originally started in 2008 (though there may be a few based on the original movies/TV shows).
I know some girls who are huge comic book fans, but 9 out of 10 fans I know got into reading comics post MCU creation, and having crushes on RDJ etc, so pretty sure most of the fic out there is going to be film verse or comic book verse written more recently than you're hoping to find ¯\_(ツ)_/¯