ANTT is the official government site for road transport in Brazil and it provides information for all destinations countrywide.
CAUTION: The website is generally unfriendly especially if you don't know Portuguese. It's both in Portuguese and English but most of the information on the English part of the website is still in Portuguese.
Luckily for the section pertaining to passenger bus travel, the English version is fully functional right now, for other services you might need to get a Portuguese speaking friend to help.
Here is the guide to help you on how to use the website
Once you're on the website click on passengers
On the next page, you can search for the various bus lines that run between various destinations across Brazil. The links to the websites of the various bus service companies will be given and you can proceed to book￼. For the companies which are approved by ANTT, their websites will contain all information as pertaining to bus fees and schedules.