Hi Wendy,
The RSPB will replace your membership card, if you say you have mislaid or lost it and if you phone up the RSPB membership department, and the telephone number is in the RSPB quarterly magazine. I can't check the number for you as I'm away from home for a few days holiday.
Regards,
Ian.
Do you mean the one to get into reserves, if so just email the RSPB or pop into your nearest reserve and they will order a replacement for you.
Here is a link to the contact page that Ian mentioned.
http://www.rspb.org.uk/contactus/
Regards Shane
My Photos in Flickr.