Alternatives to owning your own machines are renting them, and using a free (or "public") service. An example of the latter is the University of Wisconsin at Milwaukee: they list three locations with locked shredders that the public can use. These will generally be located at charity organizations or public institutions (such as libraries and universities). They will all be low-capacity, hand-fed machines, suitable for occasional use as an alternative to just trashing the paper.
For any other short-term shredding crisis, machines at all levels are available for rent. You will know the approximate volume of the stuff you want to dispose of, and the quality of it, so you'll be able to specify fairly closely the sort of machine you'll need. If you're edging up into industrial volumes or types, it is probably better to hire a mobile service with a qualified operator. Big machines are big machines and can do big damage if not properly operated; nor would you want to be liable for repair or replacement charges caused by your own unqualified people. You don't even want to think about injuries to people that don't know how to run the machines safely!