The first thing you need to decide is the kind of wood you want. Lots of people get into the plank-on-frame world with basswood, and it's not a bad material: fairly soft, easily carved, reasonably priced, and easy to find. Good hobby shops and online dealers sell it in a huge variety of sizes and shapes. Veteran plank-on-frame modelers (of whom I am NOT one; I've never built a plank-on-frame model) don't like it much, though; they find it too soft, and it's not the best wood for natural finishes. It tends to soak up finishing materials like a sponge, and many pieces have a slightly fuzzy texture that's hard to eradicate.
The next step up is cherry. It's harder, takes a beautiful finish, and can be bought in a fair variety of sizes. Model Expo sells a limited assortment of it.
Woodcraft ( www.woodcraft.com ) sells quite a variety of woods in both big boards and modelers' sizes. That site is a terrific wishbook for all sorts of things related to woodworking - many of them highly useful for modelers.
If you really want to dive head and shoulders into the plank-on-frame game, though, the place to shop for wood is The Lumber Yard, whose address scottrc provided. Those folks carry all sorts of domestic and exotic hardwoods, such as pearwood, boxwood, and (my favorite for deck planking) holly. They have the facilities to mill stock to whatever size and shape you like. Their prices aren't cheap, but they have a fine reputation.
Good luck.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.