OsCommerce Multiple Tax Rates Add On

I have an osCommerce store based out of Los Angeles, California (which has different tax rates for the state, county and cities). I need to implement an add-on to calculate the sales tax for my customers by zip code. I currently have a tax rate table in my database that was hand coded and input by a previous Developer. The rates change often and I need an easier solution or a way to change them myself on the admin side. I have an excel sheet with the zip codes and rates already. Budget is $30.