My dream is to fly with a supersonic plane. Since the Concorde is no longer active, I don't think it is possible to do this with a regular plane. So how can I get onto board of a supersonic plane? Ideally it should be in Central Europe.
There are a few companies in South Africa and I think the USA that offer rides in old military jets to wealthy tourists. Same can be had in Russia and maybe some other former Soviet republics.
South African prices for supersonic flights start at 11000 Euro.
In central Europe, laws typically prohibit supersonic flight except over open water, and even there it's severely restricted to a few narrow corridors reserved for active military aircraft (not counting military emergencies like scrambled fighters to intercept unidentified aircraft of course).
Those are your only options, as (as you said) there are no supersonic airliners left in service anywhere and other supersonic civilian aircraft have never existed (there was a plan for a supersonic business jet several years ago from Sukhoi in cooperation with I think Boeing, but it never entered even prototype stage).
http://www.kapstadt.de/jets/ (looks to be be the same company)
Fly a Russian MiG-29
There is a company MigFlug that offers flights in Russian MiG-29 fighter jets. They offer five locations within Europe, but with the prevention of supersonic flights in Europe, you may have to go for their central Russia location.
They actually only advertise near-supersonic speeds, but maybe a few rubels more get you over the threshold. Those MiGs are certainly very capable of supersonic flights.
They don't mention any prices on their web site, you have to contact them.