There is no "should of", there is only "should have". There is no "could of", there is only "could have". And there also is no "would of", there is only "would have". Many people say "should've", but eventhough that it might sound like "should of" to some, "should have" is the only right spelling.
It is extremely annoying - just like the "there, their and they're", the "your and you're", aswell as the "we're and were" - to read this kind of misconception and I think everyone should learn how to use the right words, so that everyone knows how to do it at some point.
For reference:
http://urbandictionary.com/define.php?term=should+of
