It is hard to buy a motherboard which is the core of the phone.
But it is easy to get the replacement parts on line.
You can get the original Samsung Galaxy S4 parts at ifixt.com or [http://www.witrigs.com/oem-rear-facing-camera-replacement-parts-for-samsung-galaxy-s4-gt-i9505|witrigs.com].
Ifxit offers the screen and battery replacement part of S4.
Witrigs offers complete Galaxy S4 spare parts for several versions.