Information like who in congress supported a bill before it became a law and who voted for a bill.
I assume this is a federal law. THOMAS, a service of the Library of Congress, might be a good place to start. I think it’s public but I usually get into it from work so I’m not absolutely sure. There is also something called GPO access–I can’t provide links at the moment (they are at work) but I have gotten there by putting in the bill number, if you happen to have it. You can also search the Congressional Record and Federal Register online (for US Code). If you have access to Lexis/Nexis or Westlaw that would make it easier, too.
You might be able to find the public law number or statute number from the USC. US Statutes at Large gives bill number references and sometimes leg. history cites.
If you are near a public law library (circuit court, supreme court of your state) you can call and ask a law librarian. There are some already compiled legislative histories for popular subjects and they could tell you if these are available in their library. (They are very expensive, so not a lot of libraries have them. They can run to 30 volumes–maybe more. They can also be just an assortment of loose papers, in which case you might have to look at them under supervision, if it’s even possible.)
Here is a link to research tips from law librarians. (Hey, you might not have to call one after all.)
(I have had the job of gathering Congressional Record pages for one of these published legislative histories. Lots of fun. Very interesting to see what windbags our elected officials are.)
THOMAS is indeed available online to the general public. A great resource.