Buying artificial sweeteners in Beijing

I'm diabetic and now that I'm in Beijing, I need some way of buying artificial sweeteners. I've looked in most of the local supermarkets (and my wife, who is Chinese, has also asked), but they say they don't know and haven't got any. So, can anyone suggest where I might buy some. We live in Haidian (near Xitucheng metro station) to give you an idea of the area.

I'm also aware, as a scientist, of the research that some might think of as indicating that you should not use artificial sweeteners, and I am making an informed choice to use them, thank you.