RAM & Memory Upgrades
Timetec Hynix IC Apple DDR4 2400MHz PC4-19200 SODIMM Memory Upgrade For iMac Retina 4k/5K 21.5-inch/27-inch Mid 2017 (32GB Kit(4x8GB))
DDR4 2400MHz PC4-19200 260 Pin Unbuffered Non-ECC 1.2V CL17
https://www.bizrate.co.uk/ram_memoryupgrades/timetec-hynix-ic-apple-ddr4-oid34301764689.html