The police certificate that the U.S. Embassy requires is from the Doha police department, if you're living here in Qatar. It takes approximately 2 weeks to issue the certificate and the place is located on Aziziya road (same place where you do fingerprints, etc, when a person first comes to Qatar).

You will also need a police certificate from your home country, which your embassy can assist you in obtaining. So basically you need 2 different police certs, home country and Qatar police cert.

Hope that helps.