flash cs4 is the latest and most expensive version. you can still purchase flash cs3 and unless you want to use some of the cs4 specific features, you should be good.
You will want to make sure you computer will be compatible with whichever version of Flash you decide to get. The most recent versions, CS3 and CS4, will not work on older systems. Also, if you only want to do animations, as opposed to creating code-based movies that utilize the latest coding features available, Flash 8 or earlier may be all you need, but finding where to purchase them legitimately may require some searching.