Busses are a cheap way to get around in Europe. If you don't fear a long voyage and want to come to Evoke for a more than reasonable price, this might be something for you.
There are many smaller bus companies reaching Cologne from all parts of Europe, but the most important is probably Eurolines, which has dependencies in 25 coutries, including the most Eastern Europe ones. Check out the dependence for your country here, and make sure to book your tickets early to get some possible discounts.
The busses from Eurolines, as well as most other international ones, arrive at the Central Bus Station, which is located right behind the Central Trainstation (Hauptbahnhof). Enter the trainstation building and get to the partyplace as described here.