As I use this chair myself, I can help you from experience.
Your lumbar region is already covered by the chair's integrated support, so no additional cushion is absolutely necessary.
The original cushions (e.g. Razer Head Cushion) can be purchased directly from the manufacturer and are specially designed for this purpose. There are no official alternatives from the manufacturer, but you can use universal head and lumbar cushions, as these are usually attached with elasticated straps and fit many chairs.
I have found the following as an example, but I cannot guarantee compatibility:
https://www.digitec.ch/de...