This article details information on Shippers' Associations and how to find them.
The American Institute for Shippers’ Associations, Inc. defines Shippers’ Associations as:  "Shippers' Associations generally are non-profit transportation membership cooperatives which arrange for the domestic or international shipment of members' cargo. Associations will contract for the physical movement of the cargo with motor carriers, railroads, ocean carriers, air carriers, and others. The ability to aggregate cargo and ship the collective membership cargo at favorable volume rates is the key to the existence of the modern day Shippers' Association."

Shippers’ associations allow multiple shippers to pool their volumes in order to increase shipment volumes. With a larger shipment volume, the shippers’ association may be able to negotiate volume discounts or service contracts with ocean carriers on behalf of its members.

How can I find Shipper’s Associations?
U.S. companies should evaluate all shipping options and determine which one is best suited for their situation. Shippers’ Associations are best found through trade associations for particular products (grain, footwear, alcoholic beverages, chemicals) or by searching the web. Examples of shippers’ associations include, but are not limited to:

http://www.shipgsa.com/
http://www.midwestshippers.com/
http://unitedshippers.org/
US Shippers Association

The Federal Maritime Commission (FMC) is the independent federal agency responsible for regulating the U.S. international ocean transportation system for the benefit of U.S. exporters, importers, and the U.S. consumer. At their website, they have a repository of regulations, directories of vessel operating common carriers, freight forwarders, and non-vessel operating carriers.
 
The National Customs Brokers and Freight Forwarders Association provides a listing of freight forwarders, licensed customs brokers, and other entities that provide services to U.S. firms.