Is there a catalog from which I can choose electives?
You are free to choose any full module that fit the parameters described in the question above. There are contingents available for only the Department of History and Cultural Studies (see question on contingency agreements below). Outside of this contingency agreement, you can complete any full module at any university or college of your interest, provided the parameters listed above apply and the professor/lecturer responsible for each course has agreed to your participation.
You should check the Vorlesungsverzeichnis (Course Catalog) of the respective university to look for courses and modules of your interest. The FU Vorlesungsverzeichnis can be found here. In this website, you can find the courses offered in each program at FU, which modules they are part of, the course timings and locations on campus. Humboldt Universität and Technische Universität have their own respective course catalogs you can browse through. You can also check the respective Study and Examination Regulation (SPO), to see whether the course that you want to chose stands alone or whether it is part of a module. Only fully studies module can be accredited.