Preferably one which is independent, has reviews and industry knowledge, and is not connected to travel insurance companies.
go to about.com
is a great site
Well, you can definitely take the 2nd class bus to Flores, or if you are going to Tikal there is a crossroads where you can get off and wait for a van to pass. There may also be 1st-class but I don’t know, I do recall one from Cancun to Tikal. From Flores its like 4-5 hours to Rio Dulce and 9-10 hours to Guatemala City, with everything from chicken buses to "deluxe" buses.