Not sure about your particular route but have just completed a ferry from Kenya to Lanseria stopping in Lilongwe on the way. They (being the bowser driver) reported that avgas was available in both Blantyre and Lilongwe for what it's worth to you.
Fuel Company in both places is Puma fuel which is international and my jetA experience of their service is very good. They take account or card or cash payments.
Weather once you're out of the East African corner was brilliant blue skies all the way down south although some storms around the Cape area yesterday.
Best of luck. Fly safely....