http://cancun.rutadirecta.com
If you click on the actual route number under "Listado de Rutas", it then displays the actual route on the map to the left. You can scroll in and out on the map to get a greater level of details. An example is, R-27 takes you right past the Plaza Las America's.
I also found this as well.
CANCUN BUS ROUTES
The two major routes in Cancun proper are R1 and R2. Both will take you where you want to go. R1 is the route to go downtown or to the bus station. The only real thing here to make sure of is that you grab the bus on the right side of the street to take you where you want to go. The buses on the ocean side of the street will go toward downtown, and the lagoon side buses will go toward the airport.
Major Routes
R1 - Take this route to go downtown, or to Puerto Juarez, Wal-Mart, Plaza 2000, Mercado 23 and the Cancun bus station.
R2 - Goes to Wal-Mart & Mercado 28.
Minor Routes
R15 - Goes to Wal-Mart & Mercado 28.
R27 - Goes along Tulum Ave to Plaza Las Americas where you catch the collectivo to Playa del Carmen and Tulum.
There is also Bus One, a white bus that runs in the Cancun Hotel Zone, which is the only bus with air conditioning.