The usual treatment of multiple myeloma involves one of the three alkylating agents i.e. melphalan or cyclophosphamide or chlorambucil + prednisolone. Cyclophosphamide is preferred by most oncologists due to its less toxicity. All are equally effective.
Due to the availability of alternatives and somewhat lesser use of melphalan (Alkeran - GlaxoSmithKline) and being an imported drug, it is sometimes out of stock. However some chemist shops within cancer hospitals and/or large pharmacies in metropolitan towns (Delhi, Mumbai, Calcutta, Chennai, Bangalore, Ahmedabad) do import the medicine from time to time.
In case alternatives are not to be used, your only option is to get the medicine from United Kingdom where 25 tablets of 2 mg are available for GBP 12.32. I think 5 mg tablets have been discontinued.