Some United States post offices, along with helping you process your passport application, will take a passport photo of you as well. The price for the photo is $15.
Pharmacies such as CVS or Walgreens will take your passport photo, usually processing them within the amount of time that it takes you to have your prescription filled.
College or university auxiliary services (sometimes called "student services") departments often provide passport photos to their students for a marginal fee.
Both UPS and FedEx Kinkos stores provide the opportunity for having passport photos taken to their customers.
Most large stores that have their own photo department or photography studio will take passport photos as well.