You, as the designer, determine your device's MAC address. You have options:

 - Buy a support chip that your uC talks to in order to retrieve an unique MAC address that the support chip already has in ROM (for my low-quantity products, this is what I do). 
 - Get yourself a list of legitimate MAC addresses that you specify in your firmware (requires $money$).  Google it. 
 - Tell everyone 'Good luck!' and make up your own addresses (note: If I recall correctly, there is a block of MACs that are designated to be lawlessly used, assuming the device remains only on your isolated subnet).

For the RA6M4 family, it appears that you set the MAC in the MAHR & MALR registers.

Good luck!