Depending on the specs and size requriements you have 3 options:
1) Shelves that you place the desktop case on
2) Pre-built lower end server systems
3) Use a Rackmount ATX case and build your own.
If they are going to be in a cabinet with a closed front door, you will have to take cooling into consideration. I might use something like mesh for the front door and provide exhaust so airflow can be preserved.