Northern white cedar is the ideal. Easy to find "if" you live in places like Maine and northern Michigan. It's much more difficult to find other places. Most builders use western red cedar, it's much easier to find and works fine.

Finding wood online isn't the ideal, might try www.woodfinder.com , but asking around locally at the lumber yards that specialize in hardwood lumber will often find you wood supplies you otherwise wouldn't find.

Bill H.