Are there any Mexican restaurants (not chains) in the area? They might have extra " flour" stock on hand due to the pandemic you could buy.
Reformhaus stores often have an amazing selection of products not found in a regular grocery store. Try and translate the ingredients listed on the package to German and maybe take a spoonful of the product in a little plastic container. The stores often look very old fashioned, but the staff are very knowledgeable and helpful. (the Reform movement way of eating is over 80 years old. My 90 year old mil was raised on it).
Also Alnatura and DM stock products aimed at people with diet restrictions /alternative diets but you won't get much advice there. I have seen a good selection of corn flours, but can't recall the exact names and don't have any left in my cupboard at the moment.
We have food allergies, intolerances and vegans in our immediate family: I baked/cooked every morsel we ate from scratch all the years my kids were at home. It took a lot of time but it was healthy for us and really made our traditions and family unique (in a good way) in a staunch old fashioned village 30 years ago!
I have made my own "mixes" in bulk but one needs a really big refrigerator to store it all.