You can get motherboard on two ways. Either you order it by authorized Toshiba service provider or you must search some independent electronic store where you can order it.

Both ways can be a bit complicated. If you order it by Toshiba you must give them old mainboard and to find it somewhere else is probably not so easy.

Fact is that your notebook has part number PSAM3E so you need mainboard for this model.

Do you want to exchange it alone?

Best solution is to use exact the same mainboard due to all ports there. Maybe other mainboard has some other ports and it will not be compatible with plastic notebook box. The second important thing is hardware platform. Your notebook has specific hardware platform and recovery image is designed for it so you should try to find identical mainboard.

We don’t know where do you live and it is not easy to help but use Google and try to find some online store where you can order refurbished mainboards.