Hi there,
my name is Nicho and i am currently writing my Bachelor Thesis about how fan fiction can have influence on the development of TV series...i'm especially interested in Spuffy fan fiction, because there is a lot of slash fiction about this pairing, I think even predating their relationship in the show. It's not very easy to find the very old fan fic writings though, so maybe somebody can help me out?! There needs to be no proof that the stories are actually connected to the plotline that later took place in the show, i just need some stories that are old enough so i can maybe compare them to certain episodes.
Thanks everyone!

I'm assuming you're meaning that they are paired together, because 'slash' fiction deals with same sex relationships and that's a whole different thing.  I don't know how to help you find really old stories, but some fic archives came online before the series ended.  I recommend sorting your search by date and pairing.  The show went off the air in 2003.  Fic archives that were around before then may be helpful, the biggest of course is fanfiction.net  Also, some old fic is winding up on AO3 (archive of our own.).  It's newer, but it's huge and people are putting their old stories there.

The relationship started 'officially in season 6, which aired 2001-2002, though there were hints of it in season 4 and 5. 

You may be able to google 'spuffy' and dates like 2000 or earlier and see if you get the dates of fiction...  I've put dates into google with mixed success. I cannot think of a good way to find fic that is specific to what happened in episodes, so I'm not sure you'll be able to link occurences in fic to any actual episodes... after all, how can you prove that they didn't just come up with a similar idea?

If you do this study, I'm sure we'd like to see what you come ujp with.

Good luck.

Nichdas, you could try All about Spike. That archive closed to new stories in 2006, but it is still there to read from and has some very early stories archived there. 

You can search the archive by episode, among other criteria.

http://www.allaboutspike.com/

Lots of older fic has been archived on sites like AO3 and Elysian Fields as people have left the fandom (or at least decided they no longer wish to run websites dedicated to it). Many of those stories look like they're new-ish (certainly post-2006) but have notes indicating original publication dates during the show's run. Some of those writers are still active readers (or writers in other fandoms, particularly on AO3) and may be willing to respond to questions or have recommendations for spuffy fic from that period.

The Sunnydale Memorial Fanfiction Awards (http://sunnydawards.dragonydreams.com/index.html) is an awards site that can at least give you a list of winners over past years, many of which are specified as spuffy, and that may help to date some fic, although work *can* be nominated years after it was originally written. It also has links to other (older) awards sites, which may be able to perform a similar role. Stories that appear over multiple award sites in a given year will probably be ones that were new at the time.

You may also want to check out LiveJournal's spuffy community. Someone there may have further ideas.

There were a lot of stories while the series aired that were done strictly on speculation of what was coming up next in the episode or the writers response to episodes - a good early site which can be accessed via the Way Back Machine is the Dead Poets Society and I think it is also accessible with the Buffyverse-Angel Offline Resources - 

http://offline.slayerworld.net/default_en.php?x=en

Here is an awesome link - I've checked some of the links and they all seem to work - 

http://web.archive.org/web/20050217023710/http://vampires-kiss.net/fics/

Wow. I can't believe Vamp Kiss is still up! A good source of old stories, for sure.  And yes, as someone said up thread, we'd be interested in a link to whatever you come up with for your thesis.

There were a lot of stories while the series aired that were done strictly on speculation of what was coming up next in the episode or the writers response to episodes - a good early site which can be accessed via the Way Back Machine is the Dead Poets Society and I think it is also accessible with the Buffyverse-Angel Offline Resources - 

http://offline.slayerworld.net/default_en.php?x=en

Here is an awesome link - I've checked some of the links and they all seem to work - 

http://web.archive.org/web/20050217023710/http://vampires-kiss.net/fics/