Big hardware store in Waldshut-Tiengen.
Because it's right at the border you will find a lot of swiss people here ( I also do come from Switzerland). It is huge and they have everything you need.
What I like the most in this store is that they are very friendly and helpful.
Don't expect to be overly cheap (But still cheaper then in switzerland).
At saturday can it be packed... So be willing to stand for a few minutes at the cash stands.
Something weird: you do not get a tax-return form for car equipment. read more