I always heard "shouganai" rather than "shikataganai". Looked it up and found out neither is wrong [1]. Shouganai is actually shortened from shiyouganai (using the same shi kanji as shikatagani) so that can be used to get the kanji. Although apparently the meanings are almost identical according to this answer.
[1] http://japanese.stackexchange.com/questions/8231/how-do-thes...