If not at home, trafitionally the visum is obtained in Liberville, since this seems to be (temporarily?) impossible at the moment, the next option is Pointe-Noir, or Kinchasa/Matadi, depending on your route.
I haven't heart of people having trouble getting the visa in Matadi, but if you're going trough Cabinda you need it earlier.
When I applied n Point-Noir (april/may 2004), they first told me that I should get a Resident Visa for Congo-B before they can issue the Angolan visum. After my experiences on the RN1, I was sure about one thing:"I don't want to get involved in aquiring another congolese visa" (I would rather drive back to Gabon or fly to Kin, but there where not enough pages left in my pasport for these). With the help of the Begian Consul, next day I managed to get the visa. Not something I would count on though... .