Finding parts for Silicon Graphics workstations can be challenging. These vendors specialize in vintage SGI equipment.
Ira Moser
Ira Moser maintains a collection of vintage SGI equipment salvaged from defense contractors. Direct contact is recommended for specialty items that may not be listed.
Resource: http://www.meco.org/catalog/
SGI Depot
SGI Depot offers comprehensive inventory listings and is a good resource for specialized equipment searches.