This product is either discontinued or we cannot get that from suppliers.
Synology Spare Part PSU 400W/500W_1, 400W/500W PSU Internal, Replacement Power, AC input voltage 100 - 240 V, Compatible DS2413+, Grey, 1 fan(s).
Synology Spare Part 400W/500W PSU Internal Replacement Power Supply for DS2413+ (PSU 400W/500W_1)