Sorry I'm a little late with this, but hopefully someone is still checking back. In my attempt to locate a certain fic for this posting, I remembered there was an entire thread posted earlier on with many recs that fit your time period criteria, so you may want to check out that posting. Here is a
link to the thread.
The specific fic I wanted to rec (which I can't believe wasn't mentioned) is
Unsuffer Me by Enigmaticblue. Blue is a great author and I enjoy many of her fics.
Summary: Set between BtVS S2 and S3. A chance meeting leads to an unexpected connection.
There is also a sequel (which many of us just recently learned about) and that is what brought me back to the earlier thread on this topic. The sequel takes place a couple years later, which I know is outside your time frame, but is worth reading if you've read Unsuffer Me.
Certain Dark Things by EnigmaticblueSummary: The sequel to
Unsuffer Me. Spike returns to Sunnydale, only to find the Initiative waiting for him.
A/N: In this 'verse, the events of "Lover's Walk" never happened.
Enjoy!