A-Tech 32GB (2x16GB) RAM for Apple iMac 2017 (27 inch Retina 5K & 21.5 inch Retina 4K) | DDR4 2400MHz SODIMM PC4-19200 2Rx8 1.2V 260-Pin SO-DIMM Memory Upgrade Kit at Amazon.com
Amazon.com: OWC 32GB (2 x 16GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with Mac Mini (Late 2018), 27" and 21.5" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S32P) : Everything Else
Teardown of New 4K 21.5-inch iMac Reveals Removable RAM and Modular CPU - MacRumors
64GB Memory Upgrade Kit for 21.5-inch Apple iMac 2017
iMac 21.5-inch 4K (2017) Review | Trusted Reviews
64GB RAM upgrade kit for 21.5-inch iMAC now offered by OWC - 9to5Mac
Timetec 32GB KIT(2x16GB) Compatible for Apple 2017 iMac (27-inch w/Retina 5K, 21.5-inch w/Retina 4K / Non-Retina 4K) DDR4 2400MHz PC4-19200 SODIMM MAC RAM Upgrade for iMac 18,1 / iMac 18,2 / iMac