You would get a smaller list if you ask for what companies/industries DO NOT use a mainframe computer.
In the Metro NY area - you have companies such as Morgan Stanley, Chase, Citibank, Merrill Lynch, Goldman for the Financial Sector
Accenture, Price Waterhouse Cooper for Auditors.
Coach, Dress Barn for Retailers
Bertelsman for Book Publishing
CoverGirl for Cosmetics.
The list goes on......
Many computer manufacturers product mainframe computers. I would check www.ibm.com, www.gateway.com, www.dell.com for the best deals on mainframe computers