Welcome to wiichat and enjoy your stay here.
Regarding your questions:
1. Given a Japanese Version Wii, is it possible to change its firmware to English version?
I don't think that is possible.
2. If it is possible to change then where can we get these firmware copies?
Again, not possible.
3. Is there a Wii that supports both Japanese and English game chips?
If you are referring to modchips, then I think that any Wii console can be modded, regardless of its region.
4. If there is none, right now I am staying in Japan, and I want to buy a Wii-English version, if someone here knows where in Japan can I buy this kind of Wii please do help me...right now, Wii-japanese version is only available at stores that i already visited...
I am sorry, but I am not too sure about that one.