I use Adobe Flash CS5. CS6 is newer. You could download the trial on adobe's website. However, any previous version of Flash is fine. It seriously doesn't matter which version as long as you're not creating games or anything. (I've used Flash professional 8, CS3, CS4, and CS5.5)
However, I must warn you, Flash is a horrible drawing program according to a lot of people. I personally find it perfectly fine because drawing in Flash is really really fast. It's vector based.
You could also use Photoshop if Flash is not your thing.