Official cambio's as opposed to black market ones are required to get ID. I think they have to pay some government squeeze, which is why the underground ones usually offer better rates. However some underground ones will try to pass fake bills.
It's on Paraguay right before you get to Florida. Go into the Galaria and just inside to the right is a little fur store with one of the fattest guys in BsAs sitting behind a little desk. Friendly folks, never any trucho (sp) and great rates. It's an unofficial place so no ID needed. Don't exchange on Friday as the rates always go down a bit for the weekend, tuesday and wednesday are the best.
When visiting a bank to exchange cash, be sure to bring your passport and not a photocopy. Banks require your passport. A photocopy of the page with your photo is good enough to use a credit card for shopping by not for changing money at banks.
I've created a new thread in the "Living in Argentina" forum titled "Currency Exchange Rates - Political Discussion" specifically to discuss the politics surrounding international currency exchange rates.
It's a local Buenos Aires website that is updated several times every day. They have a chart that compares the rates of all the cambios and banks, with the best buy and sell rates highlighted. Each cambio or bank name is a link to a popup window with the address, etc.
There are a number of currency exchange locations scattered through Recoleta and the Microcenter. However, after utilizing many of them at one point or another, I've come to the conclusion that the "best" place to exchange currency is the 500 block of Sarmiento, near the intersection of San Martin, one block from Florida Ave.
In this 2 block stretch of Sarmiento there are perhaps 2 dozen banks and cambios, many with signs clearly displaying their current dollar exchange rates. In my own personal experience I have determined that these numerous venues located in such close proximity to each other offer the city's best rates, no doubt spurred on by their mutual competition.
My preferred exchange house is:
Paris Cambio Corner of Reconquista and Sarmiento.
Second Location: On Sante Fe between Suipacha and Esmeralda (in the Microcenter).
There are a number of reasons why I prefer to use Paris Cambio, including:
1. They almost always have the most favorable exchange rate in the city, as demonstrated on many days by the comparison chart on www.dolarhoy.com.
2. They do not require you to present a passport or other ID to exchange money.
3. They do not close for three hours in the middle of the afternoon like many of the locations in Recoleta and elsewhere.