I purchased this adapter cable recently and it works great:
https://www.ebay.com/itm/Ugreen-USB-2-0-to-RS232-DB9-Serial-Cable-Male-A-Converter-Adapter-PL2303-for-Mac/153141429108?ssPageName=STRK:MEBIDX:IT&var=452992988415&_trksid=p2057872.m2749.l2649
It sells for $10.99 Canadian or $8.43 US...