Many sites are subscription or fee based.
Most brokerages have their own websites that are free if you know who they are. Some are regional and some are not.
Much depends on what part of the country you are seeking the load from, and what lane you are trying to stay in.
An example of this would be
www.landstarbroker.com
Some of this freight moves at a very low rates. You have to know what it costs to move your truck. This may require multiple loads if doing LTL instead of full truck load.
Davekc