I believe all but the last one are here on TSR:
Rat in a Cage by Brat
Summary: One night Buffy goes into a chat room and meets a dashing Englishman by the name of Spike. A man named William enters her world the very next day and Buffy finds herself drawn to both -- are they same guy?
There's some that are similarly themed although not exactly as you've requested. I add them in case you or someone else might think they are 'close enough':
Silently Broken by Suzee
Summary: Every night, phone in hand, Buffy Summers gives men a living soundtrack for their fantasies. He started out as just another job—but what happens when 'Spike' steps out of the phone and into her life as the very real William, her stepfather's brother?
Mixing Business with Fanfic by Sotia
Summary: In a Universe where BTVS never existed as a show (*shudders at the thought*) Elizabeth - Beth - Springs has just gotten a job with her step dad's company. On slow days - which come about a lot of the time - she reads Spuffy fanfic that seems only accessible from her work PC. What happens when William, head of the IT department and the only person seeming less than happy with her working there, decides to find out what she does in front of her computer screen all day
The Superstar And The Single Mom by Katkin
I have this one - its not online anymore. If you want a copy contact me through my contact here at TSR.
Summary: Buffy met William in a chat room three years ago when he was just an aspiring actor, as he rose to fame, he didn't tell Buffy because he wanted her to like him for who he was, and also he liked having her to himself. What happens when fame and every day life comes face to face? Can William, or 'Spike' as he is now known, convince Buffy that she's the one he really wants, and also will Buffy ever be able to forgive him for lying to her? | 
