Frankly I would buy it from China on eBay:

https://www.ebay.com/itm/Micron-8GB-16GB...

RAM Type:PC4-19200 DDR4*Min. RAM Speed:2400 MHz

Details:*Apple reports that this model shipped standard with 8 GB of "2400 MHz DDR4 onboard memory." Thankfully, however, this is not entirely accurate.

Standard RAM:8 GBMaximum RAM:32 GB*

Details:*8 GB of RAM is standard and reported as onboard with an optional upgrade to 16 GB of RAM at the time of purchase for an extra US$200. However, in a pleasant surprise, this memory actually is not onboard and can be replaced or upgraded after the initial purchase of the Mac, it's just unnecessarily complicated. This model can "unofficially" support 32 GB of RAM.

Here’s OWCs RAM options for this machine. They may ship to China

https://eshop.macsales.com/upgrades/imac...