Navigate to a free directory of HCPCS codes such as ICD9data.com. Click the link for the most recent year's HCPCS codes and select the listing of J codes. Scroll down until you see J0559. Click the link for the code to read the full description.
Pull up the FDA's National Drug Code Directory (see Resources) in your web browser. Click the link to search the directory according to the active ingredient. Type most or all the name of the drug, as provided by the description of the HCPCS code. In this example, J0559 is a 2500 unit injection of penicillin g benzathine and penicillin g procaine.
Look for an entry that matches active ingredient, method of administration and dosing the most. Scroll over to find the packaging code for the closest match. Verify other identifiable notes such as pack size and pack type against your billing notes
Write down the 11 digit NDC from the item that matches the billing and HCPCS descriptions. For example, J0559 most closely matches 54868-5932-*0, a 2 ml injection of 1200 units of bicillin.