This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
For T1 powered rockets, you shouldn't need a special liability insurance.
But some insurances explicitely exclude rocketry in the fine print of their
contracts. To be sure check this before your first launch! Ask for inclusion
or change the company if necessary!
T2 powered rockets are considered 'normal aircrafts' and the appropriate
regulations of the 'Luftrecht' rule: Aircraft up to ..kg weight
(ready-to-launch) must have a liability assurance good for DM 2,500,000
per accident. All aircrafts are insured via the "Deutscher Luftpool."
Ask the insurance company about this kind of insurance.
Probably the best alternative is to join a club where the insurance is
included in the dues.