No way to buy officially main board like a spare part. You can find like a part from other phone (with broken screen or other damages). In this situation the board will have IMEI, and you don't need to do anything.
Authorized services can order board for replacement for a warranty service or for paid service, but the price will be like new one phone. And again, you don't need to make IMEI.
From time to time (more often after rooting or bat update), the phone lost EFS data (IMEI, serial number, baseband...). There is special equipment, to fix this area from the phone memory.
The only reason to change IMEI is if the phone is reported stolen!!!