Sand dollars are sort of like the holy grail for many beachcombers in Cape Breton. Many people lament that they have "looked all their lives and have never found one." However, this does not mean they are rare--this particular creature is common in our waters, but it's just that it doesn't wash up as frequently as the other shells. (attn biology keeners: yes, you are right. the sand dollar is not a true "shell" but is in fact a "test" but we will say "shell" in the interest of communication)
Go to a beach that is known to have a sand bar for the best chance at finding these, however, know that they are very tide-dependent finds and it seems to take a special wind to wash them ashore in any great quantity--I have personally only seen "scads" of washed-ashore sand dollars a few times in my life. %90 of my sand dollar stash has been GENEROUSLY donated to me by friends, family, and oftentimes strangers who knew I used them to make pendants. Once, when I really needed them and was running low, I purchased some which were a byproduct of the fishing industry but for my pendants, I preferred the ones found "naturally" as I found they were sturdier and had more character.
Many people have heard of breaking open a sand dollar to reveal 5 doves inside--it's true, the pieces that fall out really do look like little white doves!