36-year-old Lake Havasu City resident Ryan Reavis is in custody ... this after the FBI and DEA targeted him over Mac's death. The law enforcement agencies obtained a search warrant for Reavis' home, and what they found led to his arrest.

According to investigators, drugs were found -- including prescription-only pills and marijuana. Cops say they also confiscated firearms, including a 9mm pistol and two shotguns, a homemade firearm suppressor and an arsenal of ammo.

Perhaps most important ... police found a physician's prescription pad.

Reavis was arrested and booked on fraud, drug and gun charges on $50,000 bail.

You'll recall ... Mac's alleged dealer, Cameron James Pettit, was arrested a few weeks ago for allegedly supplying Mac with deadly counterfeit oxy pills ... which cops say led to his death.

In the charging docs, prosecutors said Pettit worked with two women, who police say also played a role in supplying Mac with drugs 2 days before he died.