Areco's bus terminal is five blocks east of the central square. Chevallier runs frequent buses to/from Buenos Aires (AR$160, two hours).
A more expensive option, but a useful one if you're trying to connect to one of the area's estancias is the shuttle service operated by independent companies like Areco Bus. On weekends, the service offers roundtrip transportation between Buenos Aires and San Antonio de Areco (AR$500) with pick-up and drop-off at certain estancias, including Estancia El Ombú de Areco. Check the website for the latest itineraries and to make reservations.