I visited a message board of a reality TV show in US called "the bachlor" a couple of years ago, where the luckiest man in the world (undeserving may I say) got to pick one out of 25 gorgeous women, idealy at the end of the show, to be his wife/fiance/girlfriend (but statically the relationship never lasted a seaon--just pick any season of the year). Anyway, the bachlor was Canadian, therefore a lot of the patrons of the message board were Canadians. I noticed thier constant usage of the phrase as if it's common or acceptable in their language.
Thanks, Near There.
I did a little comparison using google, not at all scientific mind you, but it offers some measure of support for your conjecture.
+++++++++++++++++++++
Canada region:
Results 1 - 10 of about 129,000 English pages for "should of"
Population: 33,390,141 --> 0.00386%
USA region:
Results 1 - 10 of about 4,640,000 English pages for "should of".
Population: 301,139,947 --> 0.00154%
United Kingdom region:
Results 1 - 10 of about 219,000 English pages for "should of".
Population: 60,776,238 --> 0.00360%
+++++++++++++++++++++++++++++++
I've always thought that it was a mistake based on the fact that the two sounds are homophonic but maybe it's used by some speakers as a text shortcut.