Try any of the board sponsors that deal in it. Most supplement companies I've dealt with in the US are happy to send pretty much anything to Canada. Don't be surprised if it get seized however, which means you lose the money with no goods in most cases. I've done this several times with PHs (never with clen however) and it worked.
I don't know what Canada Customs would do about you personally with clen if they seized it. Guess you could always claim it wasn't yours.
PS - I'm assuming you don't know this, but openly asking for sources is a really, really bad idea. Good on you for asking about legit sites though.