Aren't most fans available as standard replacement parts? Or at least, something close enough? I once zip-tied (well, tied in with wire) a replacement CPU fan when the one I thought would fit didn't quite line up.
Dell and HP are the two major workstation brands I'm familiar with and both of them use custom fan assemblies. Search for "Dell Precision workstation fans" on eBay if you'd like to see examples.
On the other hand, ripping out the old case fan and replacing it with one held in place by sheet metal screws or glue is an option. When you are buying old hardware like this it's ok to bodge the repairs if necessary.
