+1 on FoxFifth's link. I've bought all our Mac RAM from them for over 15 years and every stick has worked perfectly because they test in Macs and focus on the Mac community.
Crucial is also very good Mac RAM. I would never buy Mac RAM from anyone other than Crucial or macsales.
OWC (macsales.com) is a US company but does some international business. That also have a large number of partner firms that handle their products outside the US:
https://www.owcdigital.com/reseller/resellers
If you are in the US I recommend ordering directly. Their ship times and service are excellent.