You don't need one, and you probably can't get one either. U.S. forces members are exempt from German registration in the first place, so there's nothing to deregister from.
A German civilian moving abroad goes to the town hall, deregisters, and walks out with a certificate. That's the document the provider's process expects. The person asking you for it is following a script, not the law.
Your orders, showing the departure date, plus a plain sentence saying you're a member of the U.S. forces under the NATO Status of Forces Agreement and therefore exempt from registration. We put that sentence in for you.
If a provider still digs in, your installation's legal office can usually write a short confirmation. It rarely comes to that.