I'd skip Morgan Street unless you're down there to go to the arch. Buffalo Brewing, Schlafly Taproom, Urban Chestnut, and Square One are all in fairly close proximity (not walking distance though). Buffalo has a small pub feel with food like mussels and buffalo sausages, Urban Chestnut has an upscale beer selection, Schlafly has a outstanding selection and the ploughman's lunch is excellent, Square One has good selection and distills their own flavored schnapps (and a good burger).
There are a whole lot of new places springing up in StL, its a hotspot for craft beer. I just know these are good ones. I want to hit Perennial and Six-Row soon. Hell I want to go to all of them.
Forgot to mention, St Louis Wine and Beermaking has a good selection of brewing stuff, plus you can get some things at one of the Friar Tucks liquor stores. If you are there Thursday evening and have a vehicle you can contact Kent at Missouri Malt Supply and have him send you a comprehensive list of the stuff he sells, he's out of a graphics design business in Fenton area and his prices are excellent.