G.Skill 16GB (2 x 8GB) DDR3 PC3-14900 1866MHz Sniper Series (9-10-9-28) Dual Channel kit (F3-1866C9D-16GSR) 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