Does anyone know where I can get elevator cable that has been removed from decommissioned elevators for an art project?

Pretty self explanatory, any length or diameter is fine.

Look in your yellow pages for a demolition company. Call them. If there's nobody in your community with a recent demo project that includes an elevator... call someone in your nearest city.
Try an elevator repair service-cables need to be replaced every so often and they would have some or know where they get rid of them. Ask building management for the number : )
I know it's not elevator cable... but maybe you can goto / phone a metal recycler and see if they have any very large wire?. or maybe if there is any shut down mills in your area there, that could be another good source for large wire.