Manaus is a city in Brazil.
Southwest towards Porto Velho and Bolivia
Highway BR319 begins in Manaus heading southwest through the Amazon. It is quickly interrupted by a shorty ferry ride from Porto da Ceasa on Manaus' far eastern edge. The ferry will cost R10.00, leaving every hour on the hour or half hour depending. Chatting with people on the ferry you may find a ride further down the road.
Disembarking in Careiro da Várzea to your left walking down the road there is a trucking facility from where you will be told trucks leave for Porto Velho. You can hang around seeing what you can find or hitch a ride to Castanho (appears on Googlemaps as "Careiro"), the last town with the last gas stations for ~450km (until the town of Realidade). There are several speed bumps leaving Careiro da Várzea that make getting a ride quite easy, and there are only so many other places traffic can be heading.