Not that hard to find but they have certain water requirements as they are african tang shell-dwelling cichlids so need harder water with a higher pH, a fine sand substrate and several shells. They are, however, perfect for a small tank and will breed readily.
Just thought I'd put in my 2-cents because minnows/danios would be a lot cheaper (and probably more hardy) than the cichlids would be, depending on what your experiment involves.