It is always legal to dig plants from land that you personally own. Using the field guide is important because you will be alerted to any rare or uncommon plants (please don't dig them) and to the habitat that the plant requires. Matching the soil drainage, soil moisture and sun/shade is very important to the success of keeping native transplants alive. Do a careful study of your home garden conditions and use a native plant reference book to make a list of plants likely to grow well in your garden, then go looking for them.

Wild plants are more difficult to transplant and more likely to die in a home garden than plants that have been nursery raised in pots. Do not attempt to transplant anything this time of year, transplant in early fall or early spring. To both preserve plants at the cottage and to have them in your garden, consider collecting only seeds or cuttings instead of digging whole plants, then grow plants in pots for later transplant to your garden.

There are a number of native plant nurseries in Southern Ontario:
Grand Moraine Growers http://www.sentex.net/~pems/index.htm
SweetGrass Gardens: http://www.execulink.com/~sggarden/

are two I like. The North American Native Plant Society has a large plant sale every spring in Toronto:
http://www.nanps.org/index.shtml

A great book for Ontario native plant gardening is The Ontario Naturalized Garden by Lorraine Johnston. The New England Wildflower Society publishes a book called Growing and Propagating Wildflowers by William Cullina which has great info about growing conditions and how to collect and germinate seed. The field guide I would recommend is Newcombs Guide to Eastern Wildflowers, it also includes vines and shrubs and has an easy to use key.

The University of Guelph Arboretum runs native plant identification and propagation workshops that are useful for beginners:
http://www.uoguelph.ca/arboretum/
And the Arboretum has a plant sale in early September with many native plants.

Good luck with your native plant garden!

Thanks for all the great advice! I have a lot of reading to do. I'm looking forward to it!