I would second what most people said above. The less things you have to buy at a wedding shop the better. They know how to gouge you! But, you do need support. I made the mistake of thinking my halter top could hold up my girls, and I was WAY wrong.
When you are trying on your dress, don't just stand there like a princess. I know you ARE a princess
but the whole day you will be sitting and standing and kneeling (maybe) and dancing and hugging and jumping around, etc. Do all of that! Make sure you are comfortable in whatever position you are in! Don't just look in the mirror with your stomach pulled in
I would suggest JC Penneys or even Walmart. You can get some from there, don't take the tags off, try it on with your dress, and return the ones you don't want. The lady at Walmart actually told me to do that.