I also looked for a fan solution on my uBITX. I tried several salvaged from computer parts, but all were too noisy I finally came up with this one:
Noctua NF-A4x20 PWM Premium-Quality Quiet 40mm Fan – $14.99 from Amazon.
A bit pricey, but it has virtually no acoustic noise and very little, if any electrical noise. Mounted on the side of my case and now everything is cool as can be.