Nightwish's Imaginaerum is now available to pre-order from their Official Webshop. There are also some bundles available, as you can see above. This version will be released on the 24th of April, and the first 300 orders receive a free badge set.
There are also some special editions available on the Nuclear Blast shop, though it seems these will not be released until the 31st of May. These editions are Steelbooks and likely to come with the option of more languages, though the default will be English. The Mail Order edition is limited to 500 numbered copies, and comes in a special box with a 1000 piece puzzle. There are no pictures of this available as yet.