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.
'Shouganai' struck me as more urban where the locals chatter at 200 words per minute. If you ever visit the back-country like Okayama Prefecture and pose one of those 'riyuu o fushigi ni omoi masu ...' (wonder why ..) questions you can still elicit both a stoic face and a 'shikataganai' in response. BTW, Jim Breen's Monash U website:
is still a good haunt for researching J-colloquialisms (ad nauseam) when and if you're in the mood. He's a retired IT prof and a friend of Jack Halpern, composer of a popular JE character dictionary.
[1] http://japanese.stackexchange.com/questions/8231/how-do-thes...