Bus services in Brazil are offered by a large number of private companies, competing within the framework of national regulations. Some travellers simply go to the bus station ("Rodoviária") on the day of departure and look around for available services. This approach often works since connections are rarely sold out, but a little bit of research beforehand is nevertheless a good idea.
In order to find out which bus services exist between two locations we recommend to use the official search form of the ANTT, the Braziliean authority for ground transportation. The website is in portuguese, but not too hard to navigate. For international travel you can also look at their list of international bus lines.
Another option is to use the search forms of the following, somewhat spammy websites:
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