It is no longer possible to obtain a witch top, or any other part of the witch 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.
Male characters cannot wear the witch top, or any other part of the witch costume. Male characters would receive a warlock costume, and if they try to put a witch top on (if they changed their gender), it will turn into a warlock top. If a female character tries to wear a witch top while changing gender, it will also turn into a warlock top.