wanted to buy: Honda block heater for a 1993 Accord EX-R.

I can't seem to locate a genuine Honda block heater. My mechanic tells me
that aftermarket block heaters may not fit properly and might pop out in
extreme cold. Apparently in southern Ontario the aftermarket block heater
might work but in northern Saskatchewan (where I'm headed this winter) the
plug might pop out. Is this any truth to this?

any help would be appreciated,