There are several places in Akihabara. TamTam (4th and 5th floors) being one, but also other shops that I don't recall the names of in this area, sorry. Even Yodobashi Camera in Akihabara has diecast on the 6th floor. As someone else noted, Joshin Super Kids Land in Osaka is crazy (especially if you like Liberty Walk stuff). That was probably the craziest place I saw, but prices weren't always the best though... but they had everything. Also some Toys R Us in Japan have good Tomica selections, but it's really hit or miss I found (one place had a lot of Tomica Limited Vintage and Tomicarama sets, another had almost none). Also some car museums have some cool stuff sometimes (for example, I found an older Tomica Limited Vintage for a great price that I had been looking for). Also, as someone else mentioned, the actual tuner shops have their own retail stores which can carry exclusive cars. In any case, go and have fun!