Home building centres carry lumber for just that...construction. The grade, quality, and dimensions are really not suitable for making quality work so I had to source out local mills in the area. Thankfully, I'm not that far (30km) from Century Mill Lumber up in Stouffville, a family run mill (150 yrs!) with a friendly staff for the newbie like me.
Prices (especially for domestic woods) are pretty competitive and on my first visit, Keith took the liberty of helping select different types of domestive lumber appropriate for my needs and took care of the shop work to plane, joint, and cut pieces to more manageable sizes.
Other Sources for GTA/Southern Ontario
• Archived page of lumber stock sources in Southern Ontario
• Exotic Woods in Burlington (Great site, but I don't like travelling west...that area is a traffic nightmare)
What to look for when you get to the mill? Well the Graham Warren book I posted about previously has a detailed chapter on various hardwood and their properties...won't get into that here. Instead, here's the bare minimum info you need to know.
I found the thickness of the stock to be most important, especially if you are going to work exclusively with hand tools as I do. 5/4 stock (meaning approximately 1 1/4" thick is best because it can be shop planed at the mill with minimal wood waste to the desired thickness of 1 1/8" - the final thickness of the shaft. 6/4 stock works as well, but then you pay for all this waste wood that needs to be removed. 4/4 stock is too thin to be used as a onepiece paddle, so even though it is more available, it's not really useable.
Next of course is the grain, which needs to be as straight as possible (although in nature nothing is perfectly straight). You may need to rummage through the stock to find boards that contain at least 3 ft of straight grain (where the grip shaft & throat of the paddle will be carved out of. Grain direction is less important in the blade. Rolf Kraiker of Blazing Paddles has a great page devoted to understanding wood grain for paddles
Finally, the length and width. Generally lumber stock dries faster on the cut edges than in the middle, so cracks can form in the wood and obviously ruin a paddle. For than reason, you want to have at least 4-6 extra inches on each terminal end of the board. For width of course, that all depends on the final width of your paddle, but again allow for an inch or so of "waste". Nothing is really wasted though, I use these scrap pieces for heating up the cottage fireplace in winter or as other wedges/scraps for other projects.
Sounds like an intimidating process at first, but I actually find it just as rewarding as using the paddle in water for the first time. To be able to visualize your work out of a rough piece of lumber is like an artist just starting to work on their canvas.