"in the past, we often used 11.0592 MHz crystals. Now, with the new X2 devices, we have to use 11.0592 MHz / 2 = 5.5296 MHz crystals to be compatible. Where can I get them in Germany? (Standard HC49/4H Package)"

Have you tried one of these manufacturers?

http://www.quarz-technik.de
http://www.petermann-technik.de

They offer a wide range of crystals and oscillators, so they probably have 5.5296 MHz, too.

Jörg