No, it is not. The correct thing to say is "would have" or "should have." These can be contracted to "would've" and "should've," which sound a lot like "would of" and "should of." Many people do not realize there is a difference.

Chloe's right - of course, it's grammatically impossible for "of" to come after would or could. However, the contracted " 've " sounds just like a neutral "of", and some native speakers absentmindedly confuse the two. Unfortunately, for some people it becomes a habit. It looks really ugly!