IMac :: Intel Dual Monitor Compatibility With Older One

So My name is Lindsey and I have been an avid apple supporter. However in the last two weeks I've come extremely close to selling everything apple I own and going back to windows. I purchased a refurbished Intel Imac through the apple store about 4 months ago. Awesome computer, everything worked great. Well I recently decided to add an additional monitor so of course I started browsing the forums and did my due diligence. I read that there were some compatibility issues with some of the older imacs and the apple 24 in led display so naturally I called my nearest apple store to find out if I could use this display.

The sales associate listened as I explained that I had a 2008 intel Imac and assured my the display would work without an adapter. Of course you guys know this is totally wrong but I was unaware of this fact at the time. SO I drove a 6 hour round trip to Dallas and paid 970 in cash for my new monitory. Get it home and realize that something is obv wrong. I call the store back and the clerk tells me I need an adapter that can be bought at Best Buy. I drive 45 min to best buy. They are sold out. Next day I drive an hour and a half to the next closest Best Buy. They dont have it either..............................