Slavery So the same way people have gotten disposable people since the dawn of time. Depending on time period this can be legal and easy or might involve contacting existing human traffickers. Assuming that magic can be used to do useful things the crones can make money and buy slaves the same as everyone else. In this case even if they are untrained in magic they can make up for it by quantity of lesser slaves.
Start a cult Use your magic to help the members a little and train them in magic, then make the most talented/successful of your members higher ranked in the cult. Steal the mana from the less talented and/or disobedient. Make it high risk, high reward. Leave the lowest level members alone so they keep breeding new members for the cult.
Basically as long as you are only targeting people that no one cares about or that volunteered you can probably exist somewhat openly, more so if you use your magic to help the general public in some way.
The Aztec empire sacrificed people for absolutely no real gain and had people consider it an honor, you should be able to do at least that much with actual magic power.