The first generation lasted from 1989 until 1991. The original model was the ES 250, and it had the same V6 engine as the Toyota Camry. This car did not sell well in the United States because it was seen as a rebadged Camry, unlike the Lexus LS series, which was based on its own platform.
The second generation lasted from 1992 until 1996. The model was now the ES 300, and it had a more robust engine and a platform shared with a Toyota not sold in the United States, which would help sales grow as Americans viewed it as a whole new car. A Coach Edition was made in 1996 with a full leather interior.
The third generation lasted from 1997 until 2001. The model was still the ES 300, but the car was longer and lighter. The suspension was an all new Adaptive Variable Suspension that adjusted the suspension on each wheel according to road conditions. This model was only available with 4 speed automatic transmission. The Coach Edition was sold from 1999 until 2001.
The fourth generation lasted from 2002 to 2006. For the first two years, the model remained the IS 300, but in 2004 some upgrades were made and it was renamed the IS 330. The engine was made bigger in the US market.
The 5th generation started in 2007. It features a 3.5 liter V6 engine. The model is now called the ES 350. The transmission is a 6 speed automatic, and the engine features variable valve timing. The base price hovers around $34,000.
These cars can be found on the used market. Lexus' are popular for their reliability, so don't expect to see one stay on the market for long. I suggest checking eBay listings, where you know when a car is going to be sold by so you don't have to worry about someone snatching it away from you.