Some medical supplies are not covered by insurance. There are several sites and organizations where people post about excess supplies, equipment, and formula they may have available. Because there are rules restricting the sale of medical equipment/supplies paid by insurance or Medicaid, most of the time you just need to pay the shipping cost to receive items.
This page also includes a list of organizations that accept extra supplies and formula as donations.
Non-Profits and Other Sites
Cecily’s Closet (San Diego, CA)