Does anybody know what is the best way to short German bonds?
CQM noted Interactive Brokers. There will be others as well that have access to the bonds directly. An easier ways to get short exposure might be through the bund futures market which will be accessible through many brokers. Even easier could be shorting a bund etf. However, remember to generally avoid inverse and leveraged etfs unless you really know what you are doing.
Finally, I couldn't help notice that this question arrived soon after this interview. While I don't disagree with the point directly, it is worth noting that many people have gone broke waiting for markets to correct themselves especially with shorts and especially when the government is intervening. While it may cost Mr. Gross very little to work this trade for the average retail investor this trade could be very expensive to put in place and to hold over time.
Interactive Brokers has access to the Euronext Bonds market, where you should be able to access German debt to short sell