Thanks for your answers!
When I started looking into Wikibase, I also found the query tool and other nice tools. But no sources, so they did not really help me in terms of development ;)
@Magnus that's nice; are you running a direct SQL call for claim[property:item]?
At the meantime I have solved it such that I simply run through all items Q1 to Q100 and parse them in case they are of the right instance (and exist). Works fine for now, performance comes later :) Now I am working on getting my data into Wikibase automatically.