Any of the insurance companies that advertise on TV or the internet are fine. You have GEICO, Progressive, Liberty Mutual, Nationwide, AllState, etc. They all sell insurance for rentals, co-ops and condos. Just call and get rate quotes.
Is this insurance for a co-op or a rental? I know in my co-op, residents are required to have insurance. In a rental, it's optional, however, it's a good idea to have rental insurance because if anything happens, the landlord's insurance policy does NOT cover anything inside your apartment.