You can buy them at American Digital:
http://www.american-digital.com/prodsit ... ame=Mitsui
I purchased a few of the Mitsui Silver 700 MByte discs to test, but haven't yet given them a thorough look. I can say that they are nothing special when it comes to initial burn quality. They aren't bad, but TY media has significantly better C1 error rates when burned on my drive.
I think Mitsui's claim to fame is long-term archival quality. Presumably that is what you are thinking about since you are interested in the gold series. (There isn't any point in paying extra for the gold reflective layer if you don't intend to keep the discs for a very long time). I haven't yet had a chance to do my own simulated aging tests, so I can't comment on their long-term durability other than pointing you to Mitsui's own longevity claims and the reputation they have established in that regard.
Physically, they seem to be well manufactured with what seems to be a tough anti-scratch coating on the top surface where CD's are most vulnerable.