So where should you as a dealer get your used car stock from?

You need to obtain them from the same source that the large used car exporters obtain them from: the Japan car auctions.  You need to buy the best vehicles available in these auctions each day, at the price they are sold for and not at any inflated FOB prices.

The used import car market in New Zealand is perhaps one of the most competitive markets in the world, and to compete in it you need an edge.  That edge is being on the front line of what is available each day at the source of most used cars sold in New Zealand, the Japan car auctions.  It requires understanding and a little skill, but if you choose a good exporter to help you through the process you can succeed.  Also, in choosing your used car exporter from Japan, try and choose one that does not have many customers from your country.  Again, if you are trying to buy online from the Japanese used car auctions from a large exporter, the large exporter faces a conflict of interest.  They may have 10 or 20 dealers all wanting to bid on the same vehicle.  They are also wanting to buy for their own stock.  Put yourself in the position of an agent for a large used car exporter in Japan, they have to get there 20 to 30 “good” cars per day for their own stock to sell to New Zealand dealers.  The stock that they do buy must sell for them in New Zealand (or any country they export to).  Where do they get their information about what is a good selling car for New Zealand?  Suddenly they have their dealers all bidding on a yellow Mazda Atenza sport.  Now what is a good car for our large exporters stock?  The car everyone wants to bid on in the auctions that day.  Who wins the competition?  The large exporters stock wins the competition.

So as a used car importer, you need to compete against these large dealers to survive.  You need to own the best available vehicles each day in the auctions.  It is a competitive world and the man with the best stock wins.

I have written this specifically thinking of the New Zealand market, but to an extent it applies to all used Japanese car importing markets that operate in a competitive market.
Get to the front line.  Choose the best stock, survive and succeed.