If you have the model correct as being a NWZ-S615F Walkman then that's a 2GB player. Here is the link to one on ebay, check it out. It's the same color and model number as described above but 2GB.
To everyone that bids i already put this in the description but im letting you know that this mp3 player has no charger!!! the battery is died out so i cant turn it on!!sorry about that and im not sure how much gigs it has...