Kaffir Lime trees are available online from Lemon Citrus Tree, Amazon.com, Nature Hills, Logee's and Four Winds Growers. All companies except Nature Hills offer different size trees; larger trees produce fruit earlier, albeit at a higher price.
It is important to note that Logee's does not ship to Texas, California, Arizona and Florida as of 2015. Lemon Citrus Tree also has state restrictions that vary by the age and size of the tree. Four Winds Growers notes that in addition to its online store, its citrus trees are available in most Northern California nurseries. It notes that if a desired tree is not available at a local nursery, the nursery can special order it for its next delivery.