One method, when a loader is not readily available, is to run off a certain length of film, and cut it there (in total darkness of course). Tape one end of the film to the cassette spool, load that into the cassette, cap the cassette ends, and spool the film in by hand. You can cut the leaders to shape at a later time, when the lights are turned on. It's a slower method. But, it works.

Loading up long lengths of film onto plastic cores and/or metal/plastic spools specifically for the film loader is definitely feasible. Someone surely has some cores or spools. But, I have always tossed them.