Jumbo probably does custom cut wood (in the large ones) but that normally means length or width, but rarely thickness (at least most of these shops don't have a planer in my experience).
Thanks for the hint about Hornbach, but as mentioned it is all thick wood, more for proper furniture.
You will not find any it is not a product that I have seen here. One option is to use parquet but the inside would be different or get 18mm thickness and split the top layer off. This is 5 or 6mm thickness on high quality parquet.
I would take any real wood plank, not plywood. I just need it to be relatively hard as I'm going for a slightly thinner wall here, since the overall dimensions are only going to be about 18cm*14cm, and I want the inner dimensions to not be tiny.
As well big bulky walls on a product that small... not the look I'm going for
Something like that to start. I'd like to be able to rebuy as well, as if this box works out I'll make a couple of more.
I will check at the FabLab in Zurich, I will use them for laser engraving the wood anyways and perhaps they have some stock on hand, as they are all about the hobby DIY projects!
I would still suggest using just the top layer of parquet. Your choice of hardwood is unlimited but you will need to get a carpenter to put it through a thickness planer or just chisel the base off and sand it. If you are cheeky go to a parquet shop and ask for a sample or if they have an old odd board. Forget Germany and the Internet get creative and practice your bad deutsch with a smile.
I know I mentioned this shop before. I have nothing to do with them and do wish their prices were a bit lower. They are still on my mind because I just visited them.
Thanks for all the helpful suggestions! The reason I don't want to go plywood like the links you posted is the appearance of the ... sides? of the wood. The part that shows the layers. I want this to be a very light/delicate box.
As for practicing the German with a smile, oh I do that a lot I'd have to find a carpetner with a planer though. Can you get untreated hardwood parquet around here? I'd like to go with a variety of stains (well really, to learn how they all look as well).