BM, Sounds like we frequented many of the same places. I usually shopped at the Akasaka and Shibuya Yamayas as well. That is where I got the great deals on the Mac Gran Reservas.
Right before I left in December of 2005, the Shibuya Yamaya had about 3 different bottlings of the Suntory Vintage Casks and the Akasaka Yamaya had the full range behind the counter with their premium wines and whiskies. Not sure what their stock is now but may be worth your while to stop in and look to see if they have any left.
Here are a few interesting ones, mostly Yamazaki, in auction right now, but many are quite expensive. The 50YO is selling for around $1300 (US)!.
http://page10.auctions.yahoo.co.jp/jp/auction/m30928523 - 15YO LE
http://page4.auctions.yahoo.co.jp/jp/auction/d61184714- 25YO LE
http://page13.auctions.yahoo.co.jp/jp/auction/r19158316- 50YO LE
Although you can't go wrong at the Helmsdale, if you get the chance I encourage you to stop in at the Mash Tun http://www.themashtun.com/. I think you will really enjoy it. I went with Duncan Elphick, formerly of the Craigellachie Hotel, shortly after the Grand opening last year and Suzuki-san was a grand host. Outside of the Suntory Vintage Casks he also had a great selection of Japanese whiskies. I specifically remember an 18YO Sherry Hakushu that was only available at the distillery that he had which was dynamite!
Hopefully my business or personal travels will bring me back around the same time and we can meet up for a few drams.