Ok Scotty! I will answer your second question first. Is there a difference in a fauna kit and a detrivore kit? Inland Aquatics explains it this way:
Quote: The Fauna Kit ($60.00) is similar to the Detritivore Kit. The difference is that each Fauna Kit does not include Bristle Worms, but instead a greater quantity of Mysis, Gammarus and other Amphipods and Copepods.
I would get the detritivore kit because of the bristle worms which are one of the most important critters you can have in your reef tank. If a fish or invert dies and you do not find it or know about it, bristle worms will make short work of the carcass.
Your first question: I do not know of any LFS that sell detritivore kits but you may ask one in your area if you can buy a cup or two of their sand if their tanks look healthy. Johnny