As of December 2022:
Northern Air is no longer a PSI testing center. Nearby Testing Centers include Red Eagle Aviation (Kalispell, MT) (406) 755-2376 OR PSI Services (Spokane, WA) (509) 443-3118.
New PSI Testing rules and software enacted; scheduling and payment is now done only online. If you get stuck during the sign-up process PSI recommends you call their customer service, (844) 704-1487.
To schedule a FAA test, you must first have your FTN, which can be found through IACRA: https://iacra.faa.gov/IACRA/Default.aspx
After retrieving your FTN, you can sign up for tests here: https://faa.psiexams.com/faa/login