It's hard to look at those poor, sad pups in pet stores and not feel the urge to 'save them.' (Which is why I don't go in any.) However, if you buy one, they will only seek to replace it, which will continue to feed the business of those producing and supplying them with pups.
In the long run, it would be doing more harm than good for the puppies in the grand scheme of things. Not to mention, you'd have a pup with questionable health and background on your hands which could lead to heartbreak for you.