where can i buy car childseat?

Where is the best place to buy a child car seat? or even rent. I am living in Higashi Koenji (not far from Shinjuku).
I may only be in Japan for 1/2 years.

try any deparment stores they should have them or try looking for baby/ children specialty shops

Cheapest places for car seats would be branches of Toys R Us - there are several scattered around Tokyo, eg Odaiba, Ikebukuro, Kameido - I don't know which would be closest for you, but there is a list of all the Japanese branches here:
http://www9.toysrus.com/our/intl/intlAsia.cfm#japan

Or try Akachan Honpo - discount store for baby and child equipment. There's a branch in Gotanda, which might be easy for you to get to.

Thanks both for the informaion i did not even know Toy R us were in Japan. It the place to start.
Thanks.

Does anyone know what is the membership fee for Akachan Honpo. Thank you

I think Akachan Honpo no longer requires you to be a member to shop there. They do still have members cards, but I think the cards are now more for loyalty points. I certainly haven't used (or been asked for) a card the last few times I went there.