Mmmm... Honestly, my favourite initial letter for girl's names is the A but if I had to choose a S name for girls...
-
Serena.
-Shailene.
-
Shirley (My favourite. Yes, I know it's a bit old-fashined but it's so pretty and cute.)
That's all. I wanted to add more but I didn't find other S names that I liked it so... Names like
Susan,
Sylvia,
Scarlett,
Sarah or
Sophia are okay but too common.