To get a Homunculus in Diablo 2: Resurrected, you must kill a monster that will drop it, but this is random. Even if you don't have a way to get Homunclus in-game, there are still ways you can increase your chances of getting Homunclus, and that is to repeatedly play the part of the monster that is most likely to drop it.
You can't get items as high-level as Homunculus on Normal difficulty. And in the Palace Cellar on Nightmare difficulty is the earliest you might get it, but you can't expect to get it from there. Among them, Baal on Nightmare difficulty is the monster most likely to drop Homunculus, then Andariel on Hell difficulty, and finally Mephisto on Hell difficulty. In conclusion, all Bosses in all Acts on Hell difficulty and all Bosses in Acts III, IV, and V on Nightmare difficulty have a high chance of dropping it.
If you feel that repeated boss battles are too boring for you and would rather play a full Act, then your best bet is Act V on Nightmare difficulty. But the speed of this method may be greatly reduced, but what if you are lucky enough? Nobody knows.