I'm looking for fic where Spike take a potion or do a spell that make him fertility.

One of kari Moukes fics has Spike drink a potion and become human for the day in order to get Buffy pregnant.

I just can't think of the name of it or find a working wbm link at the moment

I'll keep looking but hopefully someone else will know the one I mean.

This is on my ridiculously long rereads list so I'm not definite of the details but I'm pretty sure Spike takes a potion that brings out William in this so that he/Buffy can have a child.

The Happyverse series by jodyorjen
Welcome to the Happyverse, where Spike and Buffy are madly in love. Everyone else gets along...mostly.

I think the one Autezma_mete is thinking of is this...
Catching Flies by Kari Mouke
Early season four. What if Buffy had decided to use honey instead of vinegar?

Yep, that's the one. Thanks for the link, the wbm was giving me hell yesterday.