Questions About Japanese Koban Gold Pieces.

So, I've wanted one of these since I read "Shogun" by James Clavell (highly recommended, by the way). It's basically an oblong piece of gold used in feudal Japan. I was wondering if anyone here had any information to share on them, like price-level, dimensions/weight, years to look for or any auction houses (preferably in Europe) that specialize in these types of coins. I am based in Europe, so it's not really something that is seen with any regularity in the auction houses that I am familiar with.

Thanks in advance.