Razza - If you have air conditioning in your house, and if it is on all day long, then you would be best to draw air from inside the house into your grow room.

If the house is not air conditioned during the day and it gets real hot, the next coolest place to draw your air from would be under the house.

You could adapt a furnace filter to your air intake, but I wouldn't think a whole lot of insects could survive the trip through the intake fan.