The 42-year-old Johnston was arrested last week after state troopers served a search warrant at her Wasilla home. She is out on bail.
Johnston is the mother of 18-year-old Levi Johnston. Gov. Sarah Palin announced in September that her 18-year-old daughter Bristol was pregnant and Johnston was the father.
Authorities say the case began in the second week of September - a couple of weeks into Palin's campaign as Republican vice presidential candidate
- when drug investigators intercepted a package containing 179 Oxycontin pills. That led to the arrest of the suspects, who agreed to be informants.
According to the affidavit, Johnston sent a text message to one informant Oct. 1, writing: "Hey, my phones are tapped and reporters and god knows who else is always following me and the family so no privacy. I will let u no when I can go for cof."
Ten days after Johnston said there wasn't enough privacy for a drug sale, she texted again to set up a meeting at a store, according to the affidavit. The document says the informant received $800 to make a purchase, meeting investigators later with 10 pills of 80-milligram Oxycontin.
A second purchase was made the following day, authorities said. This time the informant wore a hidden camera and a microphone.
A third purchase occurred Nov. 26. The informant was wired for the transaction and this time police videotaped the meeting.
Information from: Mat-Su Valley Frontiersman, http://www.frontiersman.com