If you are going to be using more than a few I'd check online. I'm sure you can find a much better deal than at the stores around you.
Here's a couple of sites I've bought from, I haven't boought d-rings but other stuff...From this one I have ordered barrettes for making bows. She was very quick to get my order out.
www.lesbonribbon.com (I didn't see 1/2" though only 3/8" and 1" brass ones) for a pkg of 100 3/8" its 7.50
And this one I have ordered ribbon.
www.jcarolinecreative.com Her's are under belt hardware.