"Of" =/= "Have"
Contractions:
-Should have = Should've
-Could have = Could've
-Would have = Would've
The heck does "should of" even mean anyway? Think about what those two words mean individually, and then try to read them back to back in a sentence and make sense of it. Are you forming a prepositional phrase, is "should" a part of a larger grouping, or are you just typing random words based on how you think you hear them used verbally? Perhaps take a moment to contemplate this, and see if it becomes as mind jarring for you to look at as it does for me..