First off, you can purchase trees over the Internet. I have a short list of reliable companies where you can buy a seedling or sapling online. Check out my seedling supplier source page
An excellent forest nursery directory providing most tree species and covering the entire United States is maintained by the U.S. Forest Service. Also, you can find tree nurseries at most state forestry departments. You also may need some special planting tools. There are on-line specialty supply companies that provide equipment for natural resource managers. These forestry supply companies have a variety of planting equipment as well as other forestry equipment.
So, the Tree is in the Ground...
Things are pretty much out of your hands after the trees are planted. You have to leave things to Mother Nature. My experience has been that even when considering a freeze, insects, or fire, moisture is the most critical element in seedling survival for the first year or two.
Trees and Drought is a short feature explaining the effect of the lack of moisture on trees, especially seedlings and saplings. Actually, most well-established trees will tolerate drought quite well, although much depends on the species and whether they are growing on an appropriate site.