Once you have made up your mind to try your hand at carving, you will need to find a source of material, Good news most of what you need can be obtained for free from various sources.
Rural construction sites
Often a contractor starting a new building will have to remove a few trees before he can begin, the trees have to be hauled away from the site, you will be doing the contractor a favor it you can remove these trees from the site. One good tree can give you several carvings, Some exotic woods can be obtained this way depending on what part of the country you are from.
Firewood suppliers
Most major population centers have one company that supplies firewood to a portion of the area, there may be several firewood companies in you area, call one up and see what types of wood they have and what sizes they have to offer, the price should be fairly cheap and they should have some interesting types of wood for you.
Go to your local landfill and check their wood waste pile for suitable wood that you may be able to turn into something nice, this is where the wood from the construction sites ends up usually
Go out to the woods yourself
In most cases you will need a permit from your local forest office to remove wood from the forest, they are free in most cases, just make sure when you go out to the forest to find the perfect piece of wood, that you are not on private land when you begin to saw your tree.
If your looking to buy a truckload of  tree length wood 
Go to timberline magazine for information on log wholesalers and log dealers, if you are lucky you may be able to contact a logger in you area to get the best possible price on a load of logs
If you use your imagination you should be able to find wood easily and free in most cases, it doesn't take much wood to give you several weeks work. Hopefully you don't make to many mistakes or your carvings will retire as firewood.
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Where to get logs