If your are after something common, prob easier to just buy something that somebody else has imported and that your can inspect in person.

That said if you are after something more unusual you can search for it yourself in japan.

I have an account at carwebs, lets you search the auctions in japan. You can learn how to pick up auto/manual and petrol/diesel from the auction sheets fairly easy & mileage / grade, but not much else. I have bid on a couple of car's but they have sold for more than I was willing to pay. If you are looking for something around 10 years old pay attention to emission / collision standards compliance.

If you are after something fairly modern (under 3 years old) and expensive (i.e. Luxury brand), then you can some good deals out of the UK. These come with service records etc, unlike the japan cars.