Where I live ( Canada ) and in most ( all? ) States you need an refrigeration liscence to buy refrigerant gas and supplies. The cost of buying some of the specialized tools would be too costly for most home owners to own. By the laws today the unit must be repaired ( parts replaced ) and not mended and must be 100% sure of no leaks before any refrigerant gas can be put into the fridge.
Call a few local repair companies to get an idea of the cost for them to check the unit for you and then weigh the cost of repair vrs the cost of replacing the refrigerator.
Appliance Repair Aid