Quantum era is near and advanced quantum computing will pose a huge threat to the security of the current cryptographic systems in distributed energy resources (DER)rich power grids. It is necessary to prepare now to combat quantum computing attacks with serious real-world consequences in the years ahead. Recently, post-quantum cryptography (PQC) is considered as one of the major candidates of quantum attack defense strategies, and the adoption of PQC for DER systems has not been fully studied yet. This paper discusses the adoption of PQC in a standard DER network protocol (i.e., IEEE 2030.5-PQC), and proposes a real-time hardware-in-the-loop co-simulation PQC testbed consisting of a DER physical system simulation and a cyber system simulation such as DER gateways and DER management system (DERMS) server. Universal custom-made PQC client and server software are developed to meet the compliant with IEEE 2030.5-2018 standard and implemented in the DER gateways and the DERMS server, respectively. Finally, the feasibility of the proposed PQC-grade DER network is validated by using the real-time co-simulation testbed.