JoAnns had Simplicity patterns on sale for $1 over President's Day weekend
And Butterick are $2 Thurs. - Sat. At least around me; I don't know if these sales vary from place to place.
Rather than go to the trouble of buying them, look on the pattern for information about the finished garment size. Usually it's at the bottom, under all the fabric requirements.
My philosophy is that it's easier to fix something that's too big than something that's too small. So, buy the 14+ pattern, cut out the size 14, and if the dress is too loose, take it in. I'm a little hesitant to tell you to get the size 12, because that might be too small! (Keep in mind I've never seen your daughter, so this is all guess work on my part.) Do all the 'adjusting' before the dress is done, so you don't have to fool with stuff like attached interfacing, etc.
Also, if you are worried about buying too much fabric, you can always take the instructions out of the package in the store and look at the layout they are suggesting. That gives you an idea how much wiggle room you have. I just made a dress where I decided I was only going to buy 3 1/2 yards of fabric, regardless of what it called for...and it worked
. But I did have some idea of how much I was altering the length (I'm short, and patterns are for tall people), so it was an educated guess. As it turns out, I was about an inch short, so I had to fix some things when I made the hem. But it was cheaper than buying 4 1/4 yards and having a ton left over!