Hmmm heres two i know off the top of my head http://www.henryfields.ca & http://www.springgarden.ca/

Nick,
Here is a website that lists Canadian garden catalog sites. Not just seeds but all kinds of gardening needs!
http://www.gardenlist.com/Canadian.htm
Some of the ones I have used are: Gardens North, Veseys, William Damm and Stokes.
I have also ordered from Thompson and Morgan, and Seeds and More located in the States. Hope this helps.
Wendy

Nick,

You should really try Veseys. They are located next door to you in PEI and have been in business for quite a few years. I have been purchasing seeds from them for years and in the last two years have purchased plants. Never have I been disappointed. I grow mostly ornamental plants but do dilly-dally with a small veggie garden every year here in Nova Scotia. I've included a link to their site.

Here is a link that might be useful: Veseys