There is no AVGAS at Opuwo, Okongwati, or Ruacana. Your best bets are Ondangwa (North of Etosha) or Dorro Nawas. Call Wilderness Air to enquire about fuel at Dorro Nawas on +264 61 255 735. Try Ondangwa on +264 81 129 3014, +264 81 259 4707 . The Tower at Ondangwa is +264 65 703 190, if you struggle with the other numbers.
Enjoy your trip.