Instruction manual. Where can I find the instruction manual online? What I really need to know is

Question

Where can I find the instruction manual online? What I really need to know is if it can be used in a vehicle without head rests in the back seats. (I thought all boosters with backs could be used, but then I bought a different brand, read the instruction manual, and found that the seat can't be used unless the vehicle seat comes above my child's ears while in the booster seat.)