I use to buy maggots in fishing-stores or in petstores if they have some.
Then simply place the box with the maggots in a warm (not hot) location and after a few days they´ll become cocoons which hatch a few days later.
You should then gutload the flies with apple and honey.
It helps to keep the maggots in a cricket-box which is sealed with one "leg" of a pair of those rally fine, nearly invisible tights. Simply cut off one of the "legs" on both sides, seal the box and make a knot with the open end.
This way you can release them better without having much of the escape.
Also put the box in the refrigerator for a few minutes before you exchange their food, otherwise a lot of them will get lost.