There already exist at least a few random MAC address generators online.
However, as I have recently learned the hard way, MAC addresses where the first group is odd are actually multicast addresses, not to be used for regular NIC addresses.
So, if you need a random MAC address, say for configuring virtualized NICs, as I do, this generator will create exlcusively even MAC addresses, so your NICs will actually work and be able to be configured via DHCP & access the network.
If you'd like to use my MAC address generation method in your own generator, with or without ads & subscriptions making you hundreds of dollars a month, I'm fine with that.
I'm looking at you miniwebtool.com!
My method isn't particularly complex, but I've released it under my favorite license, CC-BY-SA. (see: randomMacAddress.js)
Further Reading about MAC addresses on Wikipedia.