My neighbour has Shiso growing in his front yard. It grows well in this climate because I had some that my sister from Montreal gave me. You just have to remember to water it...
I seem to recall seeing seed packets for it at Kowloon Market.
I wonder if you could give Perspectives a call? Chef Blackie's used shiso in a lot of his dishes in the past, so he may have an in on a supplier.
613-271-1800. If you're lucky you might even get Chef himself on the phone: he seems to be a fun, approachable dude if you talk to him after you have dinner there.
Oishii