If you can make use of a textbook in Spanish (in otherwords a Spanish grammar book for Native Spanish speaking students) you could try Ateneo on Santa Fe or Cuspide Libros in the same block. Also there are locations of the same stores on the pedestrian section of Lavalle.
For a Spanish book in English you could try Kel Ediciones on M T Alvear.
www.kelediciones.com go to the bottom of the opening page click on "sucursales" and choose the downtown location. The address and a map are included.
Also if you continue on Santa Fe going towards Belgrano and you pass Plaza Italia and Sarmiento about 2 blocks beyond the Plaza there is a book fair type set up. Santa Fe changes from being a one way street to becoming a 2-way street and at the beginning of this change an island is formed in the middle of Santa Fe. There are numerous discount book sellers situated together on this island. I know that they have books in English there and I know that they have textbooks but you will have to rummage through the sellers stalls in order to find what you need. Who knows? It may be cheaper to buy it there. I think that this happens in the 4200 block of Santa Fe.
Someone on this website ( I guess) recommended once Librería Huemul for Spanish text books and I was surprised because I used to learn grammar with those books at school too. I don´t have any title in particular to recommend but hopefully at the store they can help you.
Here goes the address: http://www.indextar.com/ba/public/listing/view/519?cat=329
I would say that it is pretty difficult to buy any textbooks for learning spanish here in Argentina (as well as textbooks for learning english in the USA). All these spanish schools just make xerox copies of books - that's what they call "teaching materials included". I guess you can go to UBA when their classes start and buy yourself a copy for an appropriate level.
The only set of textbooks that is published here I know of is called Voices del Sur: http://www.vocesdelsur.com.ar/shop/index.asp
(this link is to the publisher and you can buy directly from them, but many major bookstores sell it as well).