I use Obagi Tretinoin, it is prescriptive but it can be purchased online. I use a clinic but get replacement products from a company called Face The Future (I use the Obagi Nu-Derm System.) You would have to fill out a questionaire and submit pics for one of their Nurses to view.
As I said, I've been using Obagi and mix my Tretinoin 0.1% cream with Obagi's Blender. I'm amazed by my skin's transformation in 8 weeks, I look younger and my forehead lines have gone. I've heard Gel absorbs better than cream.