Rack rail kits QNAP lists per rackmount model. Sorted by how many NAS models each is listed for.

PartNAS modelsBuy
QNAP RAIL-B02119Amazon eBay
QNAP RAIL-A03-5739Amazon eBay
QNAP RAIL-S0127Amazon eBay
QNAP RAIL-S0222Search Amazon
QNAP RAIL-A01-3518Amazon
QNAP RAIL-C0113Amazon
QNAP RAIL-B0112Amazon eBay
QNAP RAIL-A02-9010Amazon eBay
QNAP RAIL-E027Amazon
QNAP RAIL-E033Amazon eBay
QNAP RAIL-H011Search Amazon
QNAP RAIL-H01-CA1Search Amazon
QNAP RAIL-H01-CM1Search Amazon