Well, since you want to be a geiko and not a maiko, the most authentic look will involve wearing a wig. You might be able to use a wig cap under the wig to help prevent irritation. Depending on how much you're willing to spend, real Japanese katsura come up on eBay every now and again but they're usually very expensive. You would also have to restyle it if you wanted the actual geiko look, since most of the wigs that are sold are actually wedding wigs. Whatever wig you buy, if you want the actual geiko style, there might be instructions for it somewhere on Immortal Geisha (and if not you could join and ask). Every so often some geiko do choose to style their own hair for special events, so you could probably also get away with that if you really wanted to avoid wearing a wig, but you'll probably want to dye your hair black. I bet you can find some temporary dyes around Halloween, but you might want to talk to your stylist for help choosing something that won't damage your hair. And of course you could add extensions to help with the length issue. Real maiko and geiko styles usually involve extensions or hair pieces anyway.