There is no better time for saving tomato seeds than at the end of the season.
Save seeds from tomatoes that are fully ripe and open pollinated. Hybrid F1 varieties won't grow true to type - like the previous harvest - so it's best to choose heirlooms or older varieties that have been around some time because of their special qualities.
Here's a link to more information about saving tomato seeds. It's very easy to do and will not only save money but you will know where the seeds have come from!
Seeds that have been saved from last season will usually grow stronger plants than seeds that have been around for two or three years. Some people store them in the fridge which helps to keep them fresh.