I'd first agree with Thomas, I'd not pick hemlock, it's heavy and often has wild grain. Cedar will work but fence stock tends to be pretty thin more like 1/2" and you really need more like 3/4" especially in a fairly weak wood like western red cedar. You should be able most anywhere to get pine, it'll work fine. It's a bit heavier but it's easy to work with, even flat cut pine works ok. Home depot most everywhere I think has western pine, it's like I said a bit heavy but even then it's easy to get a 40 lb. 17' boat. Home Depot again often has a selection of WRC, you have to sort through it to find boards that don't have a lot of knots all lined up on the same side of the board, but you can usually find some.

Might also try www.woodfinder.com Look for hardwood suppliers locally and see if they carry softwood too, many do even though they primarily sell hardwoods. You don't need a lot of wood, so even if their prices are a bit higher it won't cost much still.

Bill H.