Best answer by DRSC Media
These Nano Memory cards are only available from Huawei, and are the same size as a Nano SIM.
You can find them either at a Huawei store or directly online from most retailers, such as Amazon, etc.
Here's a link for Amazon UK, if you are located in the UK:
These new memory cards are only used on Huawei's high-end offerings, being the Mate 20/Mate 20X/Mate 20 Pro/Mate 20 RS and the recently launched P30/P30 Pro. If you own a P30 Lite, any microSD card from a well-known brand (Samsung, Sandisk, etc) is fine.