I want to understand the US and Canadian markets for small (hobby)
greenhouses.  By "small" I mean greenhouses that people buy for
installation in their yards.  Square footage would be on the order of
100-300 sq. ft.  I'm not interested in large commercial greenhouses.

Here are my primary questions:
- How many units per year have been sold in the last few years?  
- Who are the top 3-5 manufacturers in terms of market share (and what share do
they have)?  

You can see this sort of greenhouse at
http://www.texasgreenhouse.com/FreeStanding/ or
http://www.igcusa.com/greenhouses-hobby.html.  What seems to be a
fairly comprehensive list of greenhouse kit manufacturers is at
http://www.greenhousegarden.com/kit%20sellers.htm.