Fisher were made by Sanyo.
Have a look at http://cpc.farnell.com/jsp/home/homepage.jsp
http://www.chsinteractive.co.uk/
The best thing to do really is take the old ones along to a repair shop as they will have catalogues with guides showing dimensions etc and simply place the old one on this guide and select the next size below. So many different types are available. They may have some that will fit.
Failing that you could always contact Sanyo.
Tony
January 2008