Don't be disappointed if you can't find many free flash source files to download and customise for your own needs. The chances are that most of the good things available are way beyond your actionscript understanding anyway, so if you tampered with them you're likely to mess up the code unless you really concentrate hard. If you're lucky you'll find a flash tutorial which will take you through the action-script step by step, and if you're luckier you'll understand what you're doing and successfully build whatever it is in flash that you're trying to create.
The other thing not to be disappointed about is that most of the flash source files that were available for free download weren't worth downloading anyway. But if you search hard you will stumble upon some very useful tutorials which offer the fla for download. Unless you're a total beginner, If you have a basic understanding of animating in flash then you'll be able to create nicer flashes than those available for download.
The best way to learn flash is from tutorials, where you'll get the source files and you can customize the component as you wish.
You don't have to worry about learning actionscrip and the ones at tutsplus are cheap and high quality!
There are some fantasic fla source files at tutsplus - just click the banner below!
Or you can download these fla source files which i made a while ago, but perhaps you can adapt them for something.