Police said yesterday that bank records show the "Capote" star made the transactions inside a supermarket near his home.
The 46-year-old actor was found dead in his bathroom Sunday.
Law enforcement officials not authorized to talk about the evidence say on condition of anonymity that Hoffman was found with plastic envelopes of what's suspected to be heroin, and they believe he overdosed.
They say he was found with at least four dozen small packets of the drug. Police are testing the suspected heroin to determine its purity.