I don't know if you can buy the actual motherboard; you might need to buy a used PSR-E443 for cheap that has some other problem-- like a busted LCD screen or broken keys-- and cannibalize it for parts.
On the other hand, if you need to replace individual components on the motherboard, you might be able to get them from an electronics supply company that sells those types of components. Do you have the Service Manual for the PSR-E443? It should list all the individual components.