Where can I buy to replace the motherboards?

I did a cursory internet search and not much turned up. A full replacement main board may not be easy to come by. That leaves you with a handful of options:

1. Take a roll of the dice and see if you can find a used one on eBay (I wouldn’t recommend this unless you have good relations with the seller)
2. Contact Shark/Ninja and ask if they have the parts and get a quote/see if they will sell or give you one
3. See if your ninja is still under warranty and file a claim
4. Since the main board is relatively simple, determine which component has gone bad and replace it (i.e. resistors, capacitors, etc.)

I don’t recommend performing option number 4 unless you are very comfortable with electronics.

Best of luck! And remember to be safe when working with electricity and electric components.

Here is a guide for using a multi meter to test components on the PCB

Here is a guide for testing different PCB components.