yep fan is measured in sides. And getting a more powerful fan won't necessarily give you more cooling. If the fan's too strong you'll jjst get blow back as channels for the air won't be able to take the extra pressure.

So basically just buy cheap fans the right size :-)

Bear in mind that fans really don't pull much power so there's nothing stopping you wiring an extra fan into the same conenctors of an existing one.
That's what I've done for my replicators.