Taken right from thier [link=http://www.modelaircraft.org/templates/ama/faqmember.asp]website[/link].

Q. - What if I lose my membership card? Can I get a new one?
A. - Yes. Call 1-800-I FLY AMA and request one. A customer service representative will ask you a few questions and process your request. Please allow seven to ten business days to receive your duplicate card.