- Firstly, don't even think about grabbing a game from a portal and decompiling it. It's immoral and it's illegal.
- One idea might be to go to http://www.flashgamelicense.com/ or https://www.mochimedia.com/ and search for pinball games. You can then contact the developers and ask them if they'd like to reskin their game for you or supply the source files so you can do it yourself. This option may actually work out much more profitable than developing the game yourself.
- If you do a google search for "flash game source code" you will find there are many companies selling source code for Flash games, often at really low prices, and you may be able to find a pinball game somewhere in there.
- Finally, if you do decide to code the game yourself, your best option might be to try http://www.box2dflash.org/ - this physics engine will handle all of the hard maths and physics you would need to do a pinball game - you would just need to learn the API and hook up the graphics.