Read this. One thing though, it depends on laptop when buying a cooler, you need to know where is the holes underneath your laptop that feeds air to vital component, such as harddisk, CPU, GPU etc.

To be honest you be better without a cooler, just lift the laptop a few centimeter is enough to allow the laptop to breathe so it will have fresh air. Try to avoid putting it on carpeted area or on bed, it will choke the laptop. I just use the bundled free 3 fan laptop cooler but with the fan removed and turn it upside down so it acts like a lifter to feed air to laptop

At least give laptop name so we could give any suggestion. Some laptop don't give any improvement whatsoever