There are for official RAM sticks which are compatible with Synology DS1819+
|DDR4 ECC SO-DIMM|
|(EOL) D4NS2133-4G||4GB||DDR4 SO-DIMM|
|(EOL) RAMEC2133DDR4SO-16G||16GB||DDR4 ECC SO-DIMM|
You can also choose unofficial but still compatible RAM sticks from Crucial:
Crucial 16GB DDR4-2666 ECC SODIMM
DDR4 PC4-21300 • CL=19 • UNBUFFERED • ECC • DDR4-2666 • 1.2V • 2048MEG X 72 •
DS1819+ 32GB ECC RAM Upgrade
What RAM should I get. Does it even matter?
In this article you suggest
Crucial 16GB DDR4 SDRAM Memory Module – 16 GB – DDR4-2666/PC4-21300 DDR4 SDRAM – CL19-1.20 V – ECC – Unbuffered – 260-pin – SoDIMM
That product has 4 Amazon reviews.
DDR4-2666, CL19, PC4-21300, rank?
However, Amazon frequently purchased together recommendation to the DS1819+ below
Crucial 32GB Kit (16GBx2) DDR4 2400 MT/s (PC4-19200) DR x8 SODIMM 260-Pin Memory – CT2K16G4SFD824A
This product has 1,848 reviews.
DDR4-2400, CL17, PC4-19200, Dual Rank
Crucial 32GB CT2K16G4SFD824A memory comes as a KIT of two 16GB sticks. This is none ECC (Error-correcting code memory) RAM which works at slower speed around 2400 MT/s. It should work, but you loose these two important points.
I hope this helps.