In self-adaptive systems, metadata about resources in the system (e.g., services, nodes) must be dynamically published, updated, and removed. Current middleware approaches use statically configured, centralised repositories for storing and retrieving of such metadata. However, in the context of peer-to-peer (P2P) environments, we can not assume the existence of server nodes that are always available for hosting such centralised services. Thus, in our planning-based adaptation middleware, we introduce a P2P broker, which is a metadata advertisement service based on P2P technology. We use a structured P2P protocol that distributes the service metadata over a set of nodes based on service type and property information. Initial experiments show that the metadata distributes well over the nodes in the network, thus enabling scalability and robustness to node failures.