you can buy the battery here
http://cgi.ebay.co.uk/Key-Fob-Remote-Batteries-CR-2025-Renault-Megane-Laguna-/140412282035?cmd=ViewItem&pt=UK_ConsumerElectronics_Batteries_SM&hash=item20b13964b3
it should be a simple job to pop the keycard open and put it a new battery , video to sshow u how is here
http://www.youtube.com/watch?v=ke_4vdiyUTA
google for the win , took me about 30 seconds of searching to find both
google is your friend
Adam, June 2010