I'm waiting for my iMac 27" late 2015 4.0ghz i7 to arrive and I'm looking to buy a 32gb kit to upgrade. I've been browsing for an hour and I can't find any specific buyer guides or websites that sell Mac compatible memory (in the EU).
According to Apple's RAM specs, I managed to find this: PCPartPicker.com
This is what the Apple specs say:
- 1867MHz DDR3 SDRAM
Source: Install memory in an iMac