In Canada, there is Canada Bloom a garden show in Toronto, very hight end http://www.canadablooms.com
And there is Metis International garden festival http://www.jardinsmetis.com
Chelsea seem very cool!
I have done a garden for Flora international for 2 years. It is a lot of expenses and time and little of return. But it is your credibility and you can invite your clients to visit.
You do it for things like to promote your self in your news paper, you can get very good pub for free.
I would highly recommend the International Garden Festival at Chaumont-sur-Loire, France. My firm participated last year and had an amazing experience – it was great to see how 20-odd people all interpret a theme so different from one another. There was a small stipend to build the garden (if you keep it simple you can do it) otherwise you need to locate a sponser. A travel stipend was also included. http://www.chaumont-jardins.com/site/page/festival/concours_en.php