It is no longer possible to obtain the warlock legs, or any other part of the warlock costume. During the 2008 Hallowe'en event, players had to go trick-or-treating in Rimmington, Port Sarim, and Draynor Village to receive candy from certain NPCs. For this part of the costume, five wrapped candies needed to be received. Maggie would give you the item when you had all the wrapped candies.
Female characters cannot wear the warlock legs, or any other part of the warlock costume. Female characters would receive a witch costume, and if they try to put the warlock legs on (if they changed their gender), it will turn into a witch skirt. If a male character tries to wear a warlock legs while changing gender, it will also turn into a witch skirt.