You can request replacement medals using SF 180 and sending it to the National Personnel Records Center in St. Louis. If you do not have your dad's military records you can request them on the same form. Lots are missing, but it doesn't hurt to try. In Section 2 of the form write "medals and service record" or something along those lines.
There is no fee for replacement medals but it does take a while to get them.
Here is the link for the SF 180
http://www.archives.gov/research_room/obtain_copies/standard_form_180.pdf
The address for Army is:
National Personnel Records Center
Medals Section (NRPMA-M)
9700 Page Avenue
St. Louis, MO 63132-5100
For Air Force (including Army Air Corps and Army Air Force) it is:
National Personnel Records Center
Air Force Reference Branch (NRPMF)
9700 Page Avenue
St. Louis, MO 63132-5100
The address for Navy and Marines is:
Bureau of Naval Personnel
Liaison Office Room 5409
9700 Page Avenue
St. Louis, MO 63132-5100