Rajah, Short cables (<1ft) don't have to be impedance matched for average CDP to DAC connection, but your clock provides, according to description, "a very high slew rate", making it necessary to match impedance. Also, this cable normally should be >1.5m, but because of this high slew rate 1m should be fine. You can find 1m BNC-BNC 50ohm cables in Digi-Key or Amazon, like this one:
https://www.amazon.com/SDTC-Tech-Coaxial-Antenna-Connector/dp/B07Q3BJ8BV/ref=sr_1_3?keywords=50ohm+coaxial+bnc+cable+1m&qid=1577289259&sr=8-3
https://www.amazon.com/SDTC-Tech-Coaxial-Antenna-Connector/dp/B07Q3BJ8BV/ref=sr_1_3?keywords=50ohm+coaxial+bnc+cable+1m&qid=1577289259&sr=8-3