Ok folks I dont know if this has been answered before but I was looking around for a Sony Extended battery and Im finding that there are generic brands out there now for the Sony Vaio S series. 

run a google search for

SONY VAIO VGN-S S1 FS Y

and they will pop up or just go onto EBAY and run a search for VGP-BPL2 (this is the sony product number)

has anyone ever tried one of these batteries before, for the price they definately cant be beat, but im a little wary about if they are actually to spec for Sony

IMO, there is no problem when using those generic batteries for VAIO laptops but Power Management applications os Sony will detect them are third party batteries and ask you should replace with their genuine batteries.