20 year old English girl looking for an apartment in Retiro for 5 months

I'm looking for comfortable accommodation in Retiro or Recoleta in Buenos Aires for a period of 5 months, while I'm working as an intern at FALGBT. I'm clean, organised and sociable. I speak fairly good Spanish and fluent English, and would be happy to speak entirely in Spanish so that I can improve. My price range is 300$-600$ per month. I am looking for a private room with a double bed preferably, in an apartment that will allow a guest to stay at some point during my occupancy ( this would be a known friend of mine)