Get a 5-gallon bucket, iron oxide dust, aluminum powder, and a magnesium strip. These are all cheap and readily available. Into the bucket, mix the aluminum and iron oxide in a 27:80 ratio by mass, respectively.

Locate a target vehicle and place the bucket on the hood of the car. Place the magnesium strip in the mixture and light the strip. Congratulations, you've made thermite, which burns at over 2200°C (4000° in hamburger units)! The heat would cause molten iron to melt through the hood of the car and seep molten iron down into the engine block, basically totaling their car. If you place the bucket closer to the windshield, the thermite should cause a fire in the car's interior. As car fires spread really quickly, the whole vehicle would be turned to a charnel house before the fire engine arrives.

The intense heat should burn away any evidence pointing to you (provided you wear gloves), the materials are easy to gather and aren't tracked by the government, and the ease of delivery makes it possible to get to and from the crime scene without arousing suspicion (if you're dressed as a construction worker, painter, or day laborer nobody will question your actions until you physically start the thermite reaction).