If you find G1 it will be in themed fashion-stores like SPANK! or NilePerch. Since they're fashion/lifestyle items and imported rather than used/donated toys, they will be very pricey, as someone here already mentioned. Not worth buying there.. actually, I SOLD them some of mine for profit. ;>
You might find G4 Merch if you're into that.. it should be found like.. everywhere by now, I guess (haven't been over for the last 5 months, so I don't know how popular-or-not it has gotten by now).
Shopping-for-Fun is a huge part of daily life and after-school/work fun in Tokyo so there is plenty of stores/shoppingmalls/whatever.. BicCamera can easily be found by foreigners I think and they sell most toylines currently on the market in Japan.
You -can- tell her to look for Takaras, but don't get your hopes up too high.. I've been living in Tokyo on&off for 7 years and am a huge toy-nut/Kaiju-addict so I frequent a lot of used toy-stores and have seen about EVERYTHING BUT a real Takara. :,|