This one's a bit of a no-brainer, but on a whim last night I typed "sound effects" into the iTunes store search bar and got a ton of results. In some cases, CDs of hundreds of effects for less than ten bucks USD.
I grabbed a few, including one that I think you all need to know about; it's simply called "300 Sound Effects" and has a red cover, released around 1998 I think... but it has a ton of "classic" effects that you've probably heard everywhere, such as numerous familiar stock screams and the "needle scratch" sound. If you would consider these must-haves then this is the collection for you to grab.
Keep on keepin' on...