As near as I can figure, at some point between Episodes III and IV, the Empire bans the use of cloning and genetic engineering in general. (This is why the Lurrians, with their expertise at bioengineering, are captured and sold as black-market slaves in Han Solo's Revenge by Brian Daley, since IIRC their world had been quarrantined.)
Timothy Zahn says in his Heir to the Empire trilogy that the Clone Wars-era clones proved to be unstable (Luke theorizes that they were driven insane by the psychic feedback of so many identical presences in the Force) so their use was discontinued. Further, the reactions of Luke & Han upon discovering a squad of cloned Stormtroopers indicate that they have never encountered this situation before.
A majority of the other novels & comics portray Stormtroopers as non-clones as well whenever the issue of who's wearing the white armor comes up. They do not resemble Jango Fett, nor are they lobotomized drones incapable of free thinking. Indeed, they often display remarkable individuality. In Tales from the Mos Eisley Cantina, Stormtrooper Davin Felth decides that slaughtering a Sandcrawler full of innocent Jawas and murdering Owen & Beru Lars simply for being in the way of his unit's search for the droids containing the Death Star plans isn't what he joined the Imperial Army to do, so he kills his CO and defects to the Rebellion. In Kevin J. Anderson's Jedi Academy Trilogy, Kyp Durron's brother Zeph was conscripted into the ranks of the Stormtroopers as an adolescent and rigidly indoctrinated at a training facility on Carida, but he retained memories of his former life. In Barbara Hambly's Children of the Jedi, Trooper Triv Pothman grows to relish his peaceful rustic lifestyle when he is stranded for decades on a remote world, and he dreads the day the Empire comes to reclaim him. Garth Ennis wrote a short tale for a recent isue of Dark Horse Comics' Star Wars Tales of a somewhat cowardly man from a destitute Outer Rim planet who enlists in the Empire because it's easier to stand behind a tyrant than stand against him. The Crimson Empire mini-series from Dark Horse gave us two Imperial Royal Guardsmen, selected from the most elite Stormtroopers, with very individual goals. Carnor Jax sabotaged the Emperor's cloning facility in a plot to seize the Imperial Throne for himself, while Kir Kanos remains loyal to the ideals of honor and duty that were instilled in him, so he wages a one-man war against a corrupt Empire which has abandoned them. The same creative team gave us an issue of the old Marvel Star Wars comic featuring a man born on Alderaan, who became a Stromtrooper out of revenge, because he blamed Princess Leia for his homeworld's destruction. He eventually sees the truth and sacrifices himself to save her. Most interesting of all, though I have not played the game myself, Rebel agent Kyle Katarn from Lucasarts Dark Forces apparently is not only an ex-Stormtrooper but also the son of a Jedi Knight.
The Emperor was known to have access to cloning facilities after Episode III, but from all accounts he only used them for creating duplicates of himself, into which he would transfer his consciousness when his previous body wore out. It was this method that allowed him to survive his death in Return of the Jedi, but the younger, stronger cloned bodies were unstable and insane due to the meddling of Carnor Jax. The cloning cylinders were used for some special unique projects, such as the creation of Joruus C'Baoth and and a copy of Darth Maul (which was killed by Darth Vader), but it would seem that they were never used to create troops on a scale approaching anything seen in Episode II until Grand Admiral Thrawn found and reactivated the Mount Tantiss facility on Wayland. It is as yet unkown how many of these clones surved Thrawn's campaign and remained part of the Imperial Forces menacing the New Republic, but I would imagine the vast majority of them have been killed by the time of the New Jedi Order.
In conclusion, consultation of my Holocron Archives says that the Stormtroopers of Episodes IV, V, & VI are not clones, but those from the Expanded Universe fiction between 5 and 20 years after Ep. VI stand a decreasing chance of being clones.
