The range of artesanal/craft beer has certainly improved, as has the quality, over the last few years. On the down side, the local palate/market is not that educated/developed so the majority of craft brewers tend to limit themselves to Rubia (a sort of lager), Roja (nominally a 'Scotch' ale) and a negra (stout), none of which will challenge the palate. If bitter & hoppy beers are your thing, then the best beers are from Sixtofer, NAT and Chulebier - Sixtofer and NAT beers are available at, the already mentioned, Cruzat in calle Viamonte. The big plus about Cruzat is that you can buy a bottle to drink/try there, before you invest your hard earned $ in bottles to take away.
Unfortunately I'm unable to add (m)any other shops that you might visit.
If you are up for a ride out to Quilmes this weekend, there will be the Quilmes Craft Beer Festival taking place on both evenings this weekend. There are supposed to be 23 craft breweries with stands and there will be live music. On Saturday, the show kicks off at 5pm and on Sunday, I believe, it will be 7pm. The festival closes at midnight each night. Could be a good opportunity to ask the brewers where the sell their beer??
Cheers