In the "server closet" in my house, I used a built-in 16" fan for circulation. The fan was a re-purposed low voltage fan solar attic fan that I selected because of its low noise and power efficiency (less than 5 watts.)
However, for the last 10 years, computer manufacturers have been on an efficiency binge. Although the servers and equipment in the closet used to consume many hundreds of watts and produce a lot of heat, they now do the same tasks with very little waste heat and power requirements.
Or in other words, don't bother with the fan. I don't even turn mine on anymore.