As of 2015, it is possible to find Indiana county codes in a PDF file linked on the Business Tax page of the Indiana Department of Revenue website. The law governing the state's taxation is in Title 6 of the Indiana Code, published on the Indiana General Assembly website.
Departmental Notice #1 contains a list of Indiana counties, their county codes, tax types, and resident and nonresident tax rates, reports IDOR. The same document contains withholding information to assist employers in Indiana.
County codes are unlikely to change, but tax law changes with some frequency. The IDOR website provides up-to-date information about these changes on its Tax Library page. However, to see the state's actual tax code, one must visit the Laws section of the IGA website to examine Title 6 of the Indiana Code.