The vast majority of the RAM in the world is made by Samsung, SK Hynix (Hyundai Electronics), and Micron (Crucial).
https://www.statista.com/statistics/271726/global-market-share-held-by-dram-chip-vendors-since-2010/
Nearly every other brand of memory (Corsair, G.Skill, Mushkin, Kingston, etc) isn't a manufacturer. They just buy DRAM chips from one of these three manufacturers, install it on to PCBs, and sell it to you as "their" memory.
So in general, as long as you buy from a brand which has a lifetime warranty, pretty much any memory is OK since they're all using the same components. If you want "direct from the manufacturer" memory, Micron/Crucial is the easiest. They have their own retail branch, support, and memory finder tool. Here's a link to their online memory finder, if you don't want to download an executable (although I suspect the executable is more accurate).
http://www.crucial.com/usa/en/memory-info