ADA is much cheaper in Japan. They have hundreds of shops around the country which sell their products. It also means majority of the shops are not specialist like TGM for example.
So you will find lot of chinese things too.
But they are trendmakers. You will see all the latest stuff there. May will be surprised sometime how badly ADA goods are presented in some shops, but the specialist there also awesome.
Tokyo has less specialist than some other cities. In Tokyo do not miss the Aqua Forest (hard to find but worth a trip)
Anyway the only thing which stops you from buying everything is the travel fee. You can buy small things, but adding up weights to your luggage will cost you more than you saved
There's a weight limitation and if you travel back with heavy items you need to pay for that.
But if you're there do not miss the tools as they are cheap and last for a lifetime.
You can find here 5 or 6 shops which we visited a month ago. Many photos has pricing too so can help to figure out
Iaplc 2013 - a set on Flickr
what is expensive in the country is the hotel and the JR/Shinkanzen trips. All others including food, taxi etc is good.