Apple buys RAM on contract from Hynix, Micron, Samsung, and sometimes Elpida, IBM and Nanya. You cannot buy directly from any of these sources (unless you are buying 100,000 or more at a time). But you can buy from a reseller. For example, Crucial puts their brand on Micron modules (but also others, they do not exclusively use Micron even though they are owned by Micron).
What is not so important as the brand of the chips, is the sellers guarantee that the specific modules are tested and compatible with your Mac. Buying a ganaric Samsung module from a PC discounter is not much safer than any other brand generic module, other than the Samsung is less likely to be faulty than the lowest price "B" grade generics.
Samsung for example makes hundreds of different modules in different configurations, so the name Samsung is not a guarantee that the specific module you are looking at is Mac compatible.
There is no performance or speed difference between two brands of modules that are 100% compatible, because the Mac sets the speed, not the RAM module. There is no performance increase to be gained by installing "faster" modules.
Thanks
Trevor
CanadaRAM.com