internet.eplus.de is correct.
According to
this press release Blau SIMs (not only refill vouchers) are sold also at gas stations, which are supplied by Lekkerland (a company supplying gas stations and kiosks). According to
Blau's sale point search Aral, Shell and Esso gas stations, which are opened on Sundays, should sell Blau SIMs. So you could search on that page for a gas station in your proximity.