Identification tags (dog tags) are only issued for identification and casualty reporting purposes in the event of the death of a current member of the armed services. The armed services have no provisions for replacement of dog tags after someone has left the military.
There are many private companies that sell custom made, replica dog tags. You can find these companies through an internet search engine or through military/veterans magazines. Please remember that none of these companies are connected to the military or the federal government.