I can heartily recommend the following company:

Adolph Jahn & Co. GmbH
Messberg 1
20095 Hamburg
Tel. +49 40 30387838
Fax +49 40 30387839

They are used by Schleicher and have a lot of glider-specific
experience. I guess a lot of folks just use the insurance offered by
shipping companies, but I've found that Adolph Jahn is about a third
of the price. They're extremely friendly, professional, efficient,
speak polite English on the phone, and do business by e-mail if that's
your choice.

KL