On a small boat its is not just how much floatation but also where it is placed. It should be large enough and low enough to support the boat with the gunnels above water if the boat is going to have a chance to survive a capsize. In designing rightable
dinghies the floatation on each side of the boat is sized to provide buoyancy equal to the weight of the boat, crew, and gear. While the bouyant weight of a wooden boat is reduced by the floation of its hull this is more than offset by the water trapped within the capsized hull.
There are a number of cheap sources for closed cell polystyrene or closed cell Polyisocyanurate foam. I would try someone like McMasters Carr. http://www.mcmaster.com/
Jeff