My local Wallyworld just removed the USB fans from the shelves due to the seasons changing, bummed I didn't stock up on a couple more... I bought a 6" USB fan by Performance Tool at another store that is disappointing in the amount of air it moves, thinking the blades to big...I use 2 of these 4" USB fans in my 3x3 for exhaust & they do fine. Walmart - $5.99 ea. I do have a 4" Fan & Filter should I need it.