Wow, this is the first time i've ever been able to help in a help request The exact same thing happened to me too.
To get one of them batteries first you ned to find out what kind you need, then buy one (or a pack, though they last years, so one should be enough) from somewhere like here: http://www.directfoto.co.uk/shop/category/Button-Cells-Silver-Oxide.aspx I got the one I needed for 99p with no postage and packing!
If you know how to, check what type of motherboard you have (I used a programme called 'PC Wizard' to tell me all that sort of stuff, saves having to take my PC apart to see what is inside it, no idea how it knows what you have though!). Then do a google search based bit of research and see what type of CMOS battery you need based on the type of motherboard you have, then you are ready to go shopping at the link I suggested