We have bottled Guinness at Lush, but unfortunately as far as I know there's no one with Guinness on tap. If you're looking for something out of a grocery store or convenient store, The Green Tree convenient store downstairs from Lush has a few beers and may have Guinness and a couple other options, and you could check the 7-11 inbetween Propaganda and Sensations. Beyond those choices and the market in the basement of the U-Center, you may have to just pick up a case in Sanlitun or at one of the Jenny Lou's. If you do make the trip to SLT, there's a great little liquor store near to Nearby the Tree whose name escapes me at the moment. Good luck!