Are you going to do dreads or braids? and are you planning on doing the corn-row like things?
I'd say, if your hair is dark and you arent doing the corn rows, just go ahead and use your own hair, and buy some braid wefts, or look into making dreadsyourself (it really isnt too hard), using wool or synthetic hair.
if you are doing the corn rows, I'd say go with a wig, so that it doesnt't take so long to get ready when you wear the costume. Though, I would get the wig proffesionally done, and not try and do it yourself, since it is really hard to put corn-rows into a wig and make it look good