Hello everyone. ***READ BELOW***
Just to clear up, MOTHERBOARD and LOGIC BOARD are the same things, as far as iPhones are concerned.
REPLACING THE MOTHERBOARD WILL GET RID OF ACTIVATION LOCK. PUTTING AN ACTIVATION LOCKED MOTHERBOARD IN A DIFFERENT PHONE WILL STILL BE ACTIVATION LOCKED.
The only place that seems to be selling these motherboards is from Aliexpress.com (just search your iPhone type + motherboard)
After that, go to the iFixit repair guide of your specific iPhone model, labeled logic board replacement guide.
You may need to buy tools, and you can do it yourself. (recommended as its not too hard) Otherwise, you can bring it in to an Authorized repair store or just your local Tech place.
Hope this helped