Brocki as suggested, market traders especially on fleamarkets are also keen to get them....you might even get lucky on a farmers market, that they'd like to take them off your hands (provided their are clean enough to use for foodstuff)

Our library here is also keen on such bags...or perhaps a Kindergarten/KITA (for craft projects) mightt ake them off your hand...

I use the smaller ones to line the bins in the bathrooms and when the 35lt bin bag isn't quite full enough to be binned yet, it goes out on the balcony and use such smaller bags for the garbage until the Müve-bag is really full to go into the container outside.