Probably not. All South African reptiles are restricted from collecting and trade (without quite onorous permits normally only granted for research). However, the range of Namaquensis extends up into Namibia - I'm not sure what their laws are regarding chameleons.
But even if you could acquire a specimen, I wouldn't recommend them to the amateur keeper. Their habitat and behaviour is vastly different from most other large chameleons (they don't live in trees, have no prehensile tail, and live in a desert...) Though I have seen footage of one drinking after water was poured onto its snout, I'm not sure how effective conventional watering techniques would be, and who knows what specific dietary requirements they may have. Plus there would be the whole lighting issue... would a 10.0 UVB bulb be correct? And what about heat? Would they need extreme heat, or would it simply kill them in captive situations because they couldn't rely on their usual techniques for regulating heat?
Somehow I don't think these guys are going to catch on in the pet trade...
