The BLM provides much info online in a fairly standardized format. Go to a state's BLM page (Google or the BLM national home page will lead you to it), click the What We Do tab, click Recreation, click Camping. You will often find the maps there. If not, use the contact info provided on the website. I've had good luck in getting responses to very specific e-mail queries ("Can I camp at location XYZ?" not "Can I camp in your state?")
Here is an example from Arizona BLM:
http://www.blm.gov/az/st/en/prog/recreation/camping.html