Depends on the marque. There's the high-end stuff, so if you've got a BMW or Mercedes you can go to Hartge or Brabus or similar, and many other marques can be done on a more local basis. If you live in Germany but own something Italian a company called Novitec is pretty much the go-to. If you have an Alfa in the UK then Autodelta is your thing. VW specialists grow on trees in most European countries, companies like Burton Power are great with Fords, Courtenay Sport with Vauxhalls and Opels... I'm not sure there's really a big firm that covers all marques but on an individual basis there's loads of options.
