I'm afraid I don't have any good secrets to share on that front. Online purchases I've made mostly through sellers on Discogs. Some are better than others. I'm in the UK and most sellers who stock Congolese records seem to be based in France, postage can be a pain sometimes. afro7.net has some interesting 7" records from time to time.

I've been trying to get back out to Africa for a while, but plans have been put on hold. Haven't made it to Congo or DRC yet, but hoping to at least make it back to Nairobi later in the year. Melodica in Nairobi sell online but I haven't bought anything from them before.