The fastest way to get an Instamatic to reload is to ask your family and friends. Chances are someone has an Instamatic in a closet or drawer.
The most common models are the Kodak Instamatic 100, 104, the green Instamatic Hawkeye models, and the last of the line, the X-15 and X-15F. They are also the easiest to find online and in junk stores and yard sales. The Instamatic 500 is probably the coolest and best Kodak 126 camera ever made. Beautiful classic style, great German optics.
Many cameras still have an old Instamatic 126 cartridge in it you can reload. Frugal Photographer
sells the excellent Solaris brand 126 film.
Hold the cartridge and twist the ends gently. When you hear cracking, stop, check your progress and keep working at it until it falls apart.
The cartridge is sealed at the seams on the ends, but once the middle weld points are cracked, the ends usually open easily.
Some 126 cartridges are harder to open. For these, I use a thin flat screwdriver to pry the film cartridge apart.
It helps to start in the middle. There are two weld or glue spots on the bottom edge. Pry these first.
Do not worry about crooked seam cracks. You can use black tape to seal the cartridge after reloading it.
You can always use plastic model glue to repair any broken pieces.
Glue a short length of black backing paper to the inside of the cartridge back. This covers the film number window. It also helps keep the film tightly in place and gives more even reflection off the pressure plate.