There are even more sizes. Bigger is better for both lower noise and more airflow. Some micro cases use 60mm fans. More common sizes are 80mm, 92mm, and 120mm. A few cases will support 140mm fans. Some rackmount blades and drive enclosures take 40mm fans.
Measure the distance between the holes. Make yourself a simple template, and take it to your local computer shop to see what matches. You can even buy them there if they don't gouge too much on the price.