You can try Domu, which aims to serve single unit landlord/owners. You may also find some success with the larger brokerages that are primarily serving tenants.
If you are not looking for a coop commission, I would think you can find plenty of brokers willing to work with your client.
Large buildings which are rentals are not going to give you much help over the phone. Good luck, the best thing for your client is to work with someone local.
is your client looking for a rental or a property to purchase? Feel free to email me and I can give you more details on the buildings. If it is for rent, there are certain buildings that will co op with brokers.