There are plenty of Mirabelle trees in Berlin, at least two on Prenzlauer Allee between the S-Bahn bridge and Danziger Straße. Easy to spot by smashed fruit on bike path. Last week I picked a good amount on Greifswalder Strasse, close to Ernst-Thälmann-Park. Enough for five glasses of chutney.
Check Mundraub for more info on local foraging.