If you live in Wellington County, try contacting The Green Legacy program [1]. This fantastic project of the County of Wellington aims to plant over 150,000 native trees each year!
It is also important to consider areas that need not be planted, eg. where natural forest migration is already underway. For sites that are removed from natural forest migration and for shelterbelt establishment, native trees are best and climate adapted seed sources are an important consideration. The Forest Gene Conservation Association of Ontario FGCA [2] is a good resource for understanding seed source and the Society of Ecological Restoration [3] for resources on what to plant. The two organizations combine resources to put a Native Plant Resource Guide together every two years. It can be obtained by calling 1-800-667-1940. The resource guide lists most of the independent native plant nurseries in Ontario www.serontario.org/publica.htm .
The Arboretum Auxiliary holds a plant sale [4] on the second Saturday of September, 9:00 a.m. to 2:00 p.m., at which many native Ontario species are available.