Its a standard CR2032 available for ~£3 from asda and teh like. Location is next to the CPU as in this picture
Easy One Silver easy one silver Mitac 7521 7521c 7521p 7521T 7521N cmos bios battery cr2032 bare cell
You need to look underneath teh laptop. Usually there are little panels that come out allowing you to upgrade memory/disk drives etc. I would be surprised if there isn't one that lets you get to this battery.