The Meeple Mini Expansion that was released last year included the rulebook from the Summer 2018 Design Contest, and in addition cubes in black and white, meeples in all colours plus grey, and a drawstring bag.
There is not really a clear relationship between the rulebook and the components, but the components enable improvements on a lot of existing games and more importantly allow for the creation of new game experiences. The recent Contest X Design Contest generated a lot of new games using the meeples, and a bunch of other games have also been published under the MMX tag (or Meeples tag) on the wiki.
Meeples and cubes should not be hard to come by for anyone wanting to expand their Green Box on their own, but as of right now this is made even easier, by the availability of a “components only” version of the MMX on thegamecrafter.com. You should also be able to get your hands on the needed components for a reasonable price at any store that sells standard game pieces, such as Spielmaterial.de. You need 40 cubes in black and white (20 each), 7 meeples in different colours, and a drawstring bag.