We are going to be flying to Amsterdam, taking a train to Koblenz and then festival bus to RaR. We will need to buy a tent and a couple of sleepbags as its going to be impossible to bring these with us on the plane from abroad.
Where is the best/cheapest place to buy these? Should we look them up in Amsterdam or will we be able to find a camping shop in Koblenz?
This shop looks good:
Größere Kartenansicht
Shop Page
Pretty near to the Koblenz centralstation. Should work 
excellent shop. bought some parts for camping there, but maybe iit is too professioneal for your needs.
IWhen i flew with a plane, i had all stuff with me, and we just bought drinks and food in Vienna.
The shop sounds good..
Additional, as a hint..:
Last year "OBI" a kind of "home depot" shop was present near the main entrance of the Festival with a Truck.. so you would able to by some small pools, grills, coal and so on...
This helps you maybee to get some funstuff on Friday or Staturday...
But don't take this as main equiptment dealer 
If it's impossible to bring it with you I guess it's also impossible to take it back to wherever you come from?
Wouldn't it be easier (and cheaper) to send your stuff to a DHL Packstation in Koblenz, use it for RaR and send it back home?
DHL Packstation
Thanks for the info. Guess we will be able to find most of what we need in Koblenz.
Im hoping to find relatively cheap stuff and then if its worth it, after the festival, I will have it sent back home using DHL as you suggest
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