Obviously your main port of call is going to be the Official Ludwig-Walkenhorst merchandise which can be found at
Men and womens t-shirts, caps, DVDs etc.
They do offer international postage, but unfortunatley there is a high price to pay for the privilidge.
Unfortunatley the DVD/Bluray is without English subtitles for non-German speakers, but the passion and emotion shines through clearly! Have a look at the trailer ... and then order yourself a copy.
It's worth checking out auction sites such as Ebay. Various items such as autographed photographs and BVB tickets sometimes come up for sale to add to your collection. In the past Laura & Kira have kindly signed merchandise to support charities such as Wir Superhelden and auctioned them to the highest bidder.
It is currently retailing for £10.99 in the Steam store.The game is currently in 'Early Access' which means that there are still a few bugs to be ironed out but is largely completed.
Monster Block Kira!