The plausible program uses the database of MAC vendor prefixes from the Wireshark manuf file to generate MAC addresses with the first three octets corresponding to a desired vendor, and the second three octets generated at random.
plausible assumes that Wireshark is installed via package management on a Debian system. If you aren't on a Debian system, you can download the manuf DB with:
make manuf
You can choose the location of your manuf file with the -d flag.
The -l flag lists all available manufacturer codes.
The -m flag selects a manufacturer to use for generating a MAC address.