I am using a Flex 6300. I see in the DDUtil help pages that a special cable is required between the PC com port and the ACOM 2000A com port. Is this cable available commercially or is it a homebrew-only job?
they are commercially available. Check what connections IE male / Female and go for it.
Sometimes you do need in addition to a normal RS232 cable a Null modem adaptor for one end. Or a null modem cable . All available on the internet . in various lengths.
But if you wish a simple set of connectors DB9's and some wire usually you need only 3 wires and you too can brag about your solder ability .