Call Dell storage support. The only way to get one is for them to ship you one (unless you know someone else with that same system (there is a version 1.0 and 1.1 of the PV715N and each version requires it's own CDs)).

If you're a legitimate customer (read: the system is registered to your name in Dell's system), you have lifetime free support (warranty is something else than support). The CDs may not be buyable, but there are English language versions of the CDs available for technicians to send (here in the USA). Obviously I wouldn't know about a German version of the OS recovery CD. If it's still under warranty especially, you can ask them to provide you a means to reinstall the OS, whether that's having Dell send the CDs (or copies) (this may not be possible due to licensing), or they reinstall the OS for you (may need to send it off).