Where can I find dried Great Northern Beans? In a recipe I am trying to make this weekend, I saw the following:

"2 cups dried Great Northern Beans, soaked,
cooked and drained"

Where can I find these?  I could not find them at my local grocery store.  Are there any substitutions that would work well?
They are a small, white, mildly flavored bean. You can use cannellini or navy beans instead.