Well, I made my last live rock purchase for my nanos online at Premium Aquatics if that tells you anything. City Pets, Fish Ranch, or Global Fish N Pet all on the southwest side would probably be the cheapest. As far as quality goes I guess you'd just have to drive around and take a look. Maybe you'll get lucky and find a fresh shipment.
My online purchase was for Marshall Island live rock (see Vendor Feedback for pics) and it was $3.49/lb + shipping which actually wasn't too much. Most of the places around Houston will sell Fiji for about $5.00/lb.