California Sales Tax By City - OsCommerce

I would like to know if someone could take this contribution [login to view URL] and tweak it so that it looks up California Sales Taxes by city instead of zip code. I have this contribution installed on my store and it works great, but I really need it to look up taxes according to city. I say this because the California State Board of Equalization does not recommend using sales tax by zip code because a lot of cities share the same zip code and those zip codes have different tax rates.