You can get the photos in the Retiro station on the way to the office of migraciones. There is a Kodak kiosko halfway down the stairs as you go to (or come up from) the Linea C (subte) boarding platform. The cost is 15.90 for four photos, and you can use the extras when you apply for their DNIs (though that might not be necessary with the "new" system). Hopefully, the kiosko will be open if you have an early appointment at migraciones.