For specific stores, if you're going to be in Beijing we get our baby formula from Hualian, a big upscale-ish expensive supermarket chain here in Beijing. They sell American brand Land O'lakes cheese and Japanese TV dinners, so if they can manage that then I trust their formula. They always make us pay for our formula separate from the rest of the purchase and have formula salesladies guarding it so that no one defies the protocol and it is all very official. They also occasionally run Mead Johnson sponsored promotions where they'll give away stuff if you buy so much formula at once. We got a very nifty Mead Johnson playmat last week.
In Beijing the biggest baby chain store is Leyou. I think they are national, maybe in other cities besides Beijing. Their baby formula prices are comparable to Hualian, I think Hualian maybe one or two kuai cheaper. The 0-6 month formula that we use, the Enfamil, costs about between 90-100RMB for a 400g box depending on the store. Chaoyang district stores seem to be more expensive than Haidian district stores! Hualian and Leyou both sell the big brands -- Wyeth, Enfamil, Similac, and some Australian brand that has commercials on TV all the time, as well as Chinese brands like Yili and Scient. I haven't seen the SMA but they might have it, I just haven't looked since we used Enfamil for both kids.
We actually used one box of Yili formula that two of my students gave to me (it was kind of sweet, these two teenaged boys, my best students, rocked up to the house when I was on maternity leave bearing gifts -- a box of formula and a bag of sunflower seeds!) when we ran out of our regular formula one night when it was too late to run to the store. I don't think our daughter much liked the taste of the Yili, but she drank it and didn't have any problems, although we went back to our regular formula when the box was finished.