Ebay offers Apple Memory Module 32GB DDR4 2400MHz SO-DIMM 2x16GB - MP7N2G/A for $134.10 (reg. $240.22) after code free shipping.

Apply Code: RETAILKING10OFF

Product Description:
• 32GB DDR4 2400MHz
• 2x16GB
• Compatible with 27-inch iMac (mid 2017).
• SO-DIMM
• (PC4-2400)