There were a couple of international sellers of Fonic SIM cards who ship worldwide but please notice that, as described
here, after registration Fonic will send out a confirmation letter to the provided registration address and if this letter cannot be delivered your SIM will be blocked. So in addition to buying the SIM card you also need to make sure the confirmation letter will be delivered to some German address, be it a hotel or a German resident who would temporarily put your name on his mailbox.