Soon after relocating to Japan, you'll want to get some essentials like figures and Gundams. After that you'll probably want some furniture.
For most of the years spent in Japan, I got a lot of my furniture from Tokyu Hands - the department store that sells just about everything under the (rising) sun. Tokyu Hands have many stores in Tokyo so its convenient to buy stuff. I think could be because they have a hefty rent bills to pay that they sell everything at retail price.