1-Why do you want an external fan? (really important question)
-If you have heat issues, try to resolve them without resorting to an external fan as it's just hiding an imminent failure.
-If you just want it to run cooler, you can:
-take out the panel and stick a fan in front of it (homemade solution, kinda cheap)
-buy a cooler for CPU..but they are internal (best solution but not cheap and might not fit yours)
-keep your computer in a cool and dry place (simple yet efficient)
-Reapply TIM (thermal interface material or thermal paste) on the CPU (old computers needs it sometimes)
You computer was not built to overheat, if it does, it might be dirty, dusty or the fans aren't doing a good job. Check that before trying to push more dust in the computer as it will sometimes be worse!