Stanley says he has the "dope" on Blanche. From whom did he get the "dope"? scene seven

Stanley arrives and tells Stella that he has learned the truth about Blanche. He has been checking her background, and has discovered that she is no lily-white virgin. Blanche lived at the Flamingo, a hotel known for not interfering with its guests activities, but she was kicked out just the same when all of Laurel ran Blanche out of town on a rail for her own epic fornications. She didn't resign from the school but was fired before the term ended, because she had been dallying with a seventeen-year-old boy.

Stanley gets his information from the supply-man at the plant.

Our supply-man down at the plant has been going through Laurel for years and he knows all about her and everybody else in the town of Laurel knows all about her. She is as famous in Laurel as if she was the President of the United States, only she is not respected by any party! This supply-man stops at a hotel called the Flamingo.