Ok, well... I just go to Wal-Mart.
I'm about the same build as you, from your description. I'm 5'4" and about 103-105 lbs. I wear a 34A. Lately I got some padded push-up bras, but they don't look any different on me than regular ones. Here's a link to some at Wal-Mart:
Walmart.com: Apparel: Juniors: Bras & Panties They're good prices, and fit just fine so far for me.
I'm also only 17. lol But I hope that helps.