I have 2 questions. The first is does it matter if the memory in the four memory slots are from different manufacturers? What is the best memory chip for this model iMac? I need 2 x 8 memory chips.
While it would be best to always use the same brand and part number for all your memory, this can not alway be done. You have to also consider that even if you this, there is no guarantee the chips were made during the same fabrication run or that they were even made by the same manufacturer. All you can hope for this that the chips were tested to insure they meet the brands published specifications.
The best advice would be to add memory in pairs and to buy the pairs together in the same memory kit. Otherwise, make sure the store has a good return policy (just in case).
Your computer uses PC2-12800 DDR3 memory.
Buying the Apple brand is probably the most expensive way to go. On line stores such as Micro Center sell Mac memory from the brands Crucial, Centon, Kingston and Patriot. The Patriot 2x8GB kit sells for $60 US.
The specs on the RAM is:
4 slots for 204-pin PC3-12800 (1600 MHz) DDR3 SO-DIMM with a maximum of 32 Gb.
Although RAM is supposed to be electrically identical from manufacturer to manufacturer, I recommend you get RAM from a single manufacturer. There are many sources for purchasing RAM.
My personal (and professional) go-to for RAM is Data Memory Systems. I've purchased from them for many years, and my school has used them almost exclusively for at least a decade.
Disclaimer: I (and my school) are satisfied customers of DMS, and have no financial stake in the company,