The cheapest form would be Liability Only insurance, from whatever company (call around and get quotes.) Each state has a legal minimum of insurance that you must carry on your vehicle. Be warned though that it is almost NEVER enough to actually cover much of anything, and will not compensate you for damage to your vehicle - it is simply for damage to others' that you are liable for (hence the name.)
Different State vary in what local companies can offer... I would recommend checking out Progressive online because they have a system to compare with other places. Also, I've not checked into them personally, but The General .com is supposed to be a good cheap place that doesn't require any background/history. They are on commercials ALL THE TIME. E-Insurance is another one that's out there a lot... I have All State, they are really comparable in my opinion.