for cheap anime t-shirts, uniqlo is selling them already.
gundam, u can get them in yodobashi camera there is one in shinjuku, akihabara , machida and etc
if u get second hand manga they are cheap. I found one in Shimokitazawa and its all just 100yen ( $1 approx ). It is just few stops from Shinjuku station on the Odakyu Line.
And of course, there is always AKIHABARA !!!