You can start by contacting the asset mangers at various leasing companies. Citi-Capital sells (or did) their off lease lifts (that the dealers or customers did not wish to purchase at FMV) through a nationwide auction company, used to be Ritchie Bros.
There are dealers that are in the wholesale lift business across the US - do a google search - one company comes to mind is Russel Equipment out of Ohio, Santana Equipment from Illinois, etc