Rachel, your effortless style is amazing... and I'm a little jealous. You always look so incredibly stylish AND comfortable. Me? I either looks stylish OR comfortable.
Here are a few choices I discovered in many different price points! (Seriously, $15-$90... In other words, we can all afford this wardrobe staple)
Ann Taylor Long Cardigan - Sale $19.99
And if your budget is not as restricted as mine...
J. Crew Long Cardigan - $89.50
You should check out ShopStyle.com. I'm addicted! You search anything you are looking for (or a category at the least) and you can sort the results by brand, store, sale, price, color and size.