Just buy a roll ($20 something shipped)... Its much cheaper than any pre-cut stuff . And with the pre-cuts, you only have one shot to get it right, while with the rolls, you can retry it.
I got mine from here...
http://www.signwarehouse.com/c-VAVA6O1510.html
I used up the whole role on two cars. Its not too difficult to do, and after you get some practice, its actually pretty easy.