You can only buy the digital edition from GS online (the link that turbine gives you when you search for mithril edition). Not available from amazon at all (mines of moria is a different thing). Keep in mind that you apparently need a separate GS account (basically email address) for each mithril you want to buy.

FYI mithril is the suggested first purchase for anyone that wants to expand from a free to play account. It costs 19.99 and includes a number of the expansions, along with a mount and 2000 turbine points.