Citing unnamed high-ranking officials from the interior ministry, local media sources said security services were also hunting for people thought to have helped Mladic avoid justice.
Vujic Valjevo is a large Serbia company that manufacturers windows and bottled water. Sources said police were also searching houses occupied by its management.
Vidoje Vujic, the factory owner, said police took his mobile phones and seized several pictures, including one of former Bosnian Serb parliament speaker Momcilo Krajisnik who is on trial in The Hague.
Serbian media have previously reported that Mladic was hiding near Valjevo and the police searched the nearby mountain of Divcibare several years ago.
Wanted for genocide
Mladic, 66, has been wanted since 1995 by the United Nation's International Criminal Tribunal for the former Yugoslavia (ICTY) in The Hague, where Radovan Karadzic, the Bosnian Serb president, now is on trial.
The former commander of the Bosnian Serb military forces has been indicted for genocide, war crimes and crimes against humanity during the 1992-95 war in Bosnia.
He notably faces charges relating to the siege of Sarajevo, which claimed more than 10,000 lives, and for the Srebrenica massacre of about 8,000 Muslim men and boys in 1995.
Serbia's co-operation with the United Nations war crimes tribunal based in The Hague is a key condition for its closer ties with the European Union, which the new pro-Western government hopes to join by 2014.
Last month, Serbia said it had intensified its hunt for Mladic before an expected visit by Serge Brammertz, the UN tribunal's chief prosecutor.
Brammertz is visiting Croatia, Serbia and Bosnia this week and next, his office said in a statement on Monday.
"The purpose of the visits is to meet with representatives of the local authorities in charge of co-operation with the ICTY in preparation for the prosecutor's regular report to the United Nations Security Council (UNSC) in December," it said.
Brammertz will submit a written report to the UNSC in November and appear before it in December to assess co-operation with the tribunal's work.
Co-operation with the tribunal involves the arrest and transfer to The Hague of Mladic and the only other remaining fugitive war crimes suspect, Goran Hadzic, a wartime Croatian Serb leader.
Karadzic was arrested in July on a bus in Belgrade, where he had been living as an alternative medicine practitioner.