Thanks everyone. The tax and weight issues all depend on how much they cost from a Toyota dealership in Japan. If they are say, $500 a corner, then two people can bring four back and pay no tax. If we are backpacking then we won't have much luggage anyway.
I don't even need them yet (touch wood) but it was something that I wanted to look into if we are going anyway.
Seriously I would not go to a direct dealership in Japan. I would find a junking yard and get parts from there. I got all my parts from there for mine. The dealership will charge you about 7 times what it is worth.. insane!! And no, the parts will not be bad, or I would find a used car lot.. you can't miss them. Depending where you go, if you can find an american air base, there are always dealerships who can get you great deals on parts without having to pay for the parts through a dealership. I know it sounds horrible