Whether you like it or not, as a tying material, craft foam is here to stay. It’s versatile, cheap, comes in a wide range of colors and perhaps, most importantly, it floats like a cork.
Much of the stuff you get at big box stores is 2 mm thick, just perfect for many different flies. I do recommend getting the smaller sheets, as they’re easier to store and manage than the larger ones.
Most foam-body flies call for strips a hook gap in width. These are easily produced by marking the width, then using scissors to cut the strip free. A ruler and a hobby knife, along with a cutting board, make this job even faster and easier.
If you’re going to be making a lot of foam-body flies, I highly recommend a paper cutter. Even a fairly inexpensive one is capable of creating consistent-width strips at a rapid rate.