Apparently Ruger had some problems with some older locks and are offering free replacement.
This might be the place to call for a key:
Sturm, Ruger & Co., Inc.
411 Sunapee Street
Newport, NH 03773
Attn: Department LK
1-888-317-6887
Otherwise you'll have to call the main headquarters:
Ruger
Firearms Sales
Lacey Place
Southport, CT 06890
Telephone: 203-256-3860
Fax: 203-255-5201