Sainsbury's do them, worth getting a decent brand like Duracell rather than the cheaper options.At a guess it'll be a CR2032 since most boards use that. It's unlikely to be so rare that you won't be able to go out and buy one from any shop that sells button batteries when you have the existing one out.
If your system is fairly new then the Customer Care team would be more than happy to send you a new one if you get in contact with them.I've got a Ultranote reporting the CMOS battery (i.e. the little coin sized one) is failing. Putting aside the question of why on such a reasonably new laptop, does anyone know what size it takes?