Here's a thread on the subject: What meds to keep on hand?
I have my home kit, my car kit and my hiking kit - the latter two are smaller versions, containing just the things I am likely to need if an accident occurs while outside. No need to cart the 'Apotheke' around.
Many of the first-aid meds I buy in the US, as OTC stuff is more readily available - but the basics are all available here from the major pet stores: Meiko, Fressnapf, Qualipet, Zooplus, etc.
It's a good idea to ask your vet for a list of what you should have.
As posted on another thread, I highly recommend taking a 'First aid for pet owners' type course. Some vet practces, Hundeschule, or Hundevereine organize these.