I buy it from medsmex.com. BUT, I was one of the people who had my credit card info stolen after using their site. Now I generate a virtual credit card every time I need to reorder. I haven't had any problems since I started doing that.
My vitual card is through the citibank site. If you have a citibank card you can set up an account to do that with them. I'm sure there are other cards that do it also.
BTW, the tri-luma is great. It has really made a difference in my sun damage.
ETA: I just finished placing an order.