You technically dont need a center core but you should check the box that your bulk loader comes it, there may be a spare. Working without a center spool, you can roll up 100ft, or roughly a roll of film that is slightly smaller than your bulk loader's compartment. and Plop the film right into it without the center spool. There is a chance it may spool out and unwind inside, but it should work fine if you load them all in one shot and not let it sit.
Or if you want to fashion a center spool you can do it with maybe a few plastic bottle caps such as the from some thing like gatorade or vitamin water, like 3 would seem the right thickness, and glue them together, make a hole in the center, and a small slot on the side to hold the film.