Obviously, one resource to find information is the web site for whatever state you're interested in. BUT, if you want a single resource, I recommend the Guide to Sales and Use Taxes from RIA. They have a very useful separate section in each state chapter for state certificate requirements.
One other issue. As a good rule of thumb, the certificate should be from the state where the ultimate consumer receives the goods. So if A in Illinois ships to B in Florida and bills B's office in Nevada, A should get a Florida resale certificate.
Sales Tax Guy