The Game Boy Printer takes 38mm (1 ½") wide thermal paper rolls with a diameter of 30mm (1 ⅕"). The original paper from Nintendo came in three shades: white, yellow, and blue and had adhesive backing so you could stick your prints. Most of the original paper rolls you find today will have expired and print quality will be poor (even if purchased as new old stock). Standard thermal paper can be used in place of the original rolls and will give you much richer prints.
Non-adhesive Paper Options
Seiko S-951 - 38mm wide, 30mm diameter
Seiko S-950 - 38mm wide, 20mm diameter
NAP-0038-009 - 38mm wide, 30mm diameter
NAP-0038-006 - 38mm wide, 28mm diameter
TH37-30 - 37mm wide, 30mm diameter
Ultrak 499 Paper - 37mm wide, 25mm diameter