I would think most paint shops sell them, as well as hardware stores. The 3M units specifically say they are good for metal surfaces.
http://www.homedepot.com/p/3M-LeadCheck-Instant-Lead-Test-Swabs-2-Pack-LC-2SDC6/203313743
http://www.homedepot.com/p/3M-LeadCheck-Instant-Lead-Test-Swabs-2-Pack-LC-2SDC6/203313743