DEMOS one word and my only answer.
Try Game Or CEX
Try Google.
Try Amazon.com, if not there, try ebay.
You cant unless you are very inteligent but you really need a degree in mathematics, science and design and technology (D.T) maybe try waiting until you are a little older before thinking of designing your own video game, but if you wanted you could try.
Go to eBay and try and find it there because the game might be out in a different country. Get a job as a play tester for that game company. You probably will hate that job, but it is a way...
try going to www.yoyogames.com and download "game maker"
try goodfeed.webs.com
Try giantbomb.com
Try e-bay.
Try browsing an online shop that sells video games, like eBay or amazon, or try your local video game store.
Most video game stores have them, try Microplay
You can try Movie Trading Company; they have a variety of video games that are cheaper than they are in the store. Or if you don't like that option, try buying the game from a friend that has already beat it.