Celina One Stop. It's less than a mile from the dam and has an excellent tackle selection. I recommend calling ahead to make sure they have shiners. They are prone to selling out quickly. Also, be sure to specify what size of shiners you want...I ended up with only a dozen shiners out of pound being the appropriate size my first time there because I didn't think to specify what size I wanted...the others were big enough to fillet.
Best be in line early in the a.m. if you want any. I could live very nicely off just the the shiners Stephen sells...LOL. Seriously they do go quick but many times he will make a quick run down to the bridge to buy more.