The $30k Stretch Goal in the Dungeon Busters Kickstarter campaign was the Limited Edition Box. Backers got to choose whether to get the LE or the standard retail box. The LE version will not be available in stores (unless the store backed the project and chose the LE box) but Mayday did make a few extra to sell while supplies last.
From KS update #4:
"By popular demand we are happy to announce the $30 K goal, all backers of the KS project will get the Limited Edition Box version. This sleek black-box version will only be available through Mayday Games and this Kickstarter project, though of course retailers who back this project will also be getting this version, so there will be a few out there after the project funds and is fulfilled."
From the KS campaign FAQ:
"The Limited Edition box will never be available through retailers or online or through distribution, we are only offering it through the KS project and then the remaining few copies through our website or directly at conventions later. It is just for you backers who want something special. If you prefer the other box then that is totally fine too, we will ask you for your preference for each copy of the game you pledge for in the survey that we send out. For example, if you back for 2 copies of the game and want one LE version and one standard version, that is totally ok. If you want 2 copies of one version, that is also fine."