Determine what type of material it is that needs proofreading. Often, what documents need proofreading may only need editing. This can only be determined after a casual look at the document and a brief talk with the recipient to find out what their needs and expectations are for the material.
Discuss with the client or recipient to find out what their needs for the material are. You may find that you may need more or less resources after your discussion with them.
Talk to and network with others in the industry to find resources for proofreaders. Message boards, chat rooms and writer's websites are great places that will list what resources are frequently used by proofreaders.
Sign up at websites that offer frequent updates on industry materials for proofreaders. Again, writer's websites and similar places are great for locating this type of information.
Use the services of a proofreader yourself first as a client, then find what resources they use as a proofreader. This works especially well if you are a company representative and are looking for someone to fill a position as a freelance proofreader.