I live in Warburg, Westfalen between Kassel and Paderborn.  I use 30%
hydrogen peroxide to remove stains from antique porcelain.  Right now,
I buy it at the local pharmacies. I pay about 27 Euros for a 5 liter
bottle. I can use about 30 liters at a time.  Can I buy it elsewhere
more cheaply? Is it available in an even higher concentration? I am
fluent in German.

1. With these quantities that you are using, you may get into troubles
with authorities - acetonperoxide was used in London sub bombing, in
the Reid shoe bombing attempt and is the usual material found in Hamas
suicide belts.
2. 50% H2O2 is comercial, you can buy it from Fluka/Aldrich
3.Your price is not very high - but you could try to search for a bulk
commodity chemical producer - if you can buy+use drums of peroxide.
Hoechst, Lonza, etc.
4. Less expensive and easier to handle substitute can be urea-peroxide
 ("solid peroxide"), sodium perborate or sodium percarbonate
5. There are always hazard shipment charges and transportation fees
involved - 30% peroxide is truck-only transport, no air

Thank you for the information.  You have given me several good
starting points, and at the same time made it clear that perhaps my
current situation is the best for my needs.