I would start with some inexpensive wood first, to give yourself a feel for the process. Maple, poplar, anything cheap, even some walnut. Depending on the size of call you want to make, a blank about 1 1/2 inches square to 2 inches square and 3 to 5 inches long is good to start with. You can always cut off the ends. After that, it's just a matter of an idea, and starting to hack away at it. You'll make mistakes, ruin a few blanks, but you will get it. Then you can go for some nice wood. I would start at Home Depot or look for wood suppliers in your area. Sawmills, that kind of thing. You should be able to find someone around you that has some wood to buy. I would stay away from the big wood working stores. They are crazy expensive and the wood isn't always dry.
Al @ THO Game Calls