It is no longer possible to obtain a warlock top, 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, six popcorn balls needed to be received. Maggie would give you the item when you had all the popcorn balls.
Female characters cannot wear the warlock top, or any other part of the warlock costume. Female characters would receive a witch costume, and if they try to put a warlock top on (if they changed their gender), it will turn into a witch top. If a male character wears a warlock top while changing their gender to female, it will also turn into a witch top.