Black and white film? It's very hard, but doable. Probably comparable difficulty to building your own computer out of logic ICs. You'll buy cellulose acetate stock, a bunch of chemicals, and build a coating machine yourself. You'll also need some chemical engineering skills. Your film might be comparable to that from the 1950s.

Alternatively, you can buy a film factory when it gets shut down, but these tend to produce film in large batches, and it might be difficult to repair the machines. People have done this before.

Color film? Basically impossible. Probably comparable difficulty to building your own ICs. Only a handful of companies were ever successful at it.