although the answer from @JuanCarlosCoto was already accepted, I still must disagree on one point
..Bear in mind that whatever external fans you may feasibly buy and
install will provide much less efficient cooling than an internal
An external fan, providing that is is not a blow-fan or fan pad , but an exhaust fan type, can be extremely effective - in fact much more than an internal one due to a much bigger facet surface and possibility to work in much higher speeds .
Exhaust type fans ( sometimes called Vacuum Air Extracting fans ) , as opposed to blow-fans or pads, work on a different concept. they are actually sucking heat from the regular exhaust grill , instead of blowing air inside ( or in case of the pads - blowing air on the surfaces of the body ).
Typology examples from google images ( but searching for the above terms will yield the same for your favorite search engine )
Doing that - they are actually taking advantage of the integral / original aerodynamic and airflow design of the laptop, causing the air to enter and flow over the heat-sink / the processor / GPU in a ,much higher volume / minute capacity ( CMq/sec ) all that through the original air ducts .
Several of my laptops have a desktop CPU combined with metal body and and are really hot at times , in fact - so hot that I was more than once inflected with a heat burn !
All the cooling pads I have tried were somewhat of a hoax , never had any real influence ( 3-5 degrees Celsius ) - except making noise .
But when I found exhaust fans - the effect was extreme .
In my tests , on an HP 8560p , after activating an exhaust fan the temperature on the integrated CPU sensor dropped 30 degrees in 20 sec ( 70's to 40's ) with immediate effect ( meaning the drop started in 1-2 seconds !! )
The second drop took some more time ( about a minute ) but it was so efficient , that the fan had to be stopped due to over - cooling now !
The test was made with a 10,000 max RPM external exhaust fan with 2 automatic adjustable speeds and 125.15CFM ( cubic feet / minute ) or 3.54 CCM ( Cubic Meter / minute ) sized 165×80×48MM 1.9W and cost 8$ !!
Another advantage is that these type of fans do not insert dust into your laptop like blow fans, but actually sucking the dust out !
And the best part - they are really cheap, and much more portable than the pads .