There were 4 generations of the Valiant in its 17 year lifespan. The first generation lasted from 1960 to 1962. It was sold as a 2 or 4 door sedan, a 4 door wagon with 2 or 3 seats, or as a 2 door coupe. The Valiant was the same car as the Dodge Lancer. Although they appeared similar to the Chevrolet Corvair, which was their closest competitor, they were in fact very different cars mechanically. The Valiant was less outrageous in its engineering, and suffered far fewer mechanical problems as a result.
The second generation Plymouth Valiant was sold from 1963 to 1966. It was sold as a 2 door convertible, 2 door hardtop, 4 door sedan, and 4 door station wagon. In 1964 the V8 engine was added as an option to replace the standard slant 6.
The third generation Plymouth Valiant was sold from 1967 to 1973. The convertible body type was dropped, as was the wagon. The Valiant Barracuda trim was spun off into its own model name, the Barracuda, which would become one of the most well-known models that Plymouth would ever make. These Valiants were very similar to the Dodge Dart, and the two dominated the American compact car market.
The fourth generation only lasted for the 1974, 1975, and 1976 model years. It was only sold as a 2 door hardtop and 4 door sedan. It became even more like the Dart, switching over the the Dodge's wheelbase in 1974. Luxury versions of the Valiant were called the Valiant Brougham. This was the same basic car as the Dodge Dart Special Edition.
I was once a fan of a little band from Seattle named Nirvana, so I have to mention that Kurt Cobain drove a 1963 Plymouth Valiant for pretty much his entire adult life. It was the car registered and insured to his name when he decided to take his own life in 1994.
These cars are slowly making their way into the collector car market. It seems that interest in the Valiant grows every year as people who cannot afford the big Plymouth muscle cars with Hemi or 440 engines decide to buy a lower priced Chrysler alternative from Dodge or Plymouth. The best place to buy a Valiant in any condition is usually going to be eBay. There are parts listings on there as well.