This plugin allows you to provide your customers with manual tax calculation. You can configure this plugin and choose, whether you will use fixed rate or country rate for your store.
Using “fixed rate” option you can specify tax rate for each product category in your store, if needed. Using “country/state/zip” option you can specify tax rate for a particular country, state or zip.
Please find more information about how to install plugins here.
The plugin is provided absolutely for free and includes source code.