@Tommy01 You can basically use any TP-Link power line AV600 or better powerline adapter to extend your system. I also advise to update the firmware on all your powerline devices to the latest HW version available. Your powerline AV500 kit will after this is completed become and act as an AV600 kit. All AV500 TP-Link devices are actually AV600 capable after the firmware has been updated.
Please note, if you buy AV1000, your AV1000 devices will transmit AV600 speed (similar if you choose aV1200 devices), higher speed powerline adapters adapt to the lowest AV speed you currently use (another reason why updating the firmware is a great idea). You can update the firmware on your powerline devices using the tpPLC utility provided by TP-Link.