You can purchase pre-crushed graham cracker crumbs from the grocery store, but if you can't seem to find them, you can crush them yourself with a rolling pin and large ziptop bag (or pulse them in a food processor). You will need about 1 whole sleeve, or 7-8 whole graham crackers to yield 1 cup crushed.
Sweetened condensed milk is a must when making magic bars. It works as the binding agent, caramelizing as it cooks with the other ingredients and helping everything stick together. Do not skip or substitute this ingredient; after all, it's the main source of the magic.