I liked shopping in DenDen town in Osaka better than Akihabara. DenDen town is all stretched out in a straight line, while Akihabara is all piled on top of itself - much harder to navigate.
We got our 860s from Au (Au by KDDI is what it says on the bag), which is a cell phone store. We saw them on a PriceJapane page back in September for ~8,000 yen off the then going price. We paid 48000 yen or so, with a colleague of my friend in Japan picking them up for us.
When I was there in October, used 860 in Sofmap were running 47500 yen, 760's for 44500, and 750's for about 38000.
Good things to look for while you're there are accessories - I got my remote for 1800 yen used, a Pilot Pentopia extensible stylus made especially for the Z, etc. (And the camera, which was around 7000 or so, expensive but a nice toy.
The Remote makes the Z as an MP3 player, and the stylus if far better than any shipped with a Z. (The metal stylus that shipped with the 5500 is a close second, but extensible rules - my favorite sylus to use is the one from my touchscreen Libretto which is much longer than a PDA stylus, but completely un-portable ;_; )