In The Dark Knight, how did the Joker get and set up all the oil drums without being found out?

Presumably he used trucks

On a more serious note… (Why so serious?)

Buying them probably would not have raised any flags drums of oil are not really a controlled substance, stealing them may have raised flags but flags ultimately dont matter if no one connects the dots in time.

It is possible that the warehouse was already being used to store the oil and the Joker merely set the bombs

They weren't oil, they were diesel.

Dent’s chair wouldn’t move—stuck on a raised board, probably. Dent shifted all his weight to one side and . . . that was a mistake. He began to topple. It was too late to regain his balance, and he fell into one of the barrels, knocking it over, falling on top of it and slipping to the floor. The concrete was cold against his cheek, then it was cold and wet; the top of the barrel had dropped open, and diesel fuel was spilling out.

_{^{The Dark Knight: Official Novelisation}}

Assuming Gotham City is anything like New York City, those can be easily purchased by contacting a fuel delivery company and giving them money. Note that his goons also managed to load hundreds from the dockside onto the ferries.