Your questionreminds me the question what is the shoes size for an orphan. Please obtain more details as size, water volume, plastic or steel, new or 2nd hand (used), is it for long term storage or for daily use, do you have problem of algae, Etc, or what your budget.
In general, you can find lots of them all over Germany. Maybe even free of charge, from the food industry. Just look for water storage tanks in the Internet.