The NDC for the drugs you are billing should be obtained directly from the package or invoice of purchase. If you need to view a CMS NDC-HCPCS Crosswalk, it can be viewed through a Medicare Part B carrier (West Virginia), Palmetto GBA's fee schedule under ASP Drug Pricing Files. It is updated quarterly.
If there are other sources for this information, I will also be interested in knowing. Representatives of our State Medicaid program state that "NDC numbers never change", but I have found the requirement for NDC numbers on claims for reimbursement to be a real challenge here.