Did the shop confirm it is a Malawi trout?... The image is not that clear.
If it is a Champsochromis caeruleus they are very cool, but get large to the point of 13"+ so to me we are taking an 8' long tank....
It terms of rarity, you can find them for sale but you will have to do some looking and should expect to order online versus a local pickup.
I came close to purchasing one but came to terms with the fact that they really need a larger tank than what I could provide and so I passed.. maybe someday!
Look them up on the web and you'll see some beautiful colors, they really are quite sleek and magnificent.