You can probably get them from somewhere that sells "button cells", like boots, currys or halfords and do them yourself.
After putting new ones in you might have to re register them with the car - the owners manual should tell you how to do this. It is different for different cars
yes i bought some of ebay and they worked perfect.and easy to fit as will,the batt is about the size of a watch one but usual if you search for a ford key batt you should find it.or you could buy it from them themselves.