This list of major credit report agencies includes, where available, links to the agencies' online dispute procedures.
However, even when such links are available, it is generally preferable to initiate the dispute via letter, and not via the company's own form. This toolkit includes a sample letter and guidelines here.
>>ChoicePoint: Full File Disclosure
>>ChoiceTrust: Review Your Fact Act Disclosure Reports
General Information Services
>>HireRight: Filing a Consumer Report Dispute