I had Brachyplatystoma flavicans other wise know as a Brachyplatystoma rousseauxii. I had to get rid of him because he was small and was probably going to be eaten by my peacock bass. This is the fish you have pictured. These fish get very large. Fishbase has their max size listed as 192 cm (around 6').
The two fish you are talking about are entirely different species. The Brachyplatystoma filamentosum (common name Kumakuma) is one of the largest fish in the amazon. Fishbase has its max length listed as 360cm (about 12') and a max weight of 200 kgs!!!!!!!!!!!!!! It is a true monster. It has been reported that it has eaten people.
If you are looking for flavicans then contact John from ECZS. I know he has a few small juveniles in the 4-5" range, and he can probably get them in bigger if you request.