Have you tried checking marriage records in your county of birth to see if you can find any women with the last name of Lamb, around the age of your birthmother, who might have married in that area? The married name might lead you to her current location.
http://www.adoptionforums.com
http://www.adoptionchat.com
http://www.adoptionlists.com
http://www.adoption.com
http://www.adopting.org
http://www.adoptionregistry.com