I'm in the commercial cleaning industry, and we manage about 10 million sqft worth of space. At this point, we obtain a lot of our accounts via word of mouth, referrals, etc. When first starting the company, it was cold calling, visiting facilities, and hoping that you found someone who was unhappy with their current service and/or cost. Property managers value a service provider who can cover a couple of their needs at once. If you could reach out to the clients you have been providing exterior detailing to, and approach them about servicing their interiors too, that may work out. They'd rather deal with you for both services instead of having to contact two separate providers.
A major downside to commercial cleaning is the low profit margin. We pull a good chunk of our revenue by offering floor and carpet services. If you're more keen on sticking with smaller facilities, starting something in floor services is always a good option (I say smaller facilities, because a lot of the time, the larger facilities will work floor/carpet services into their contract agreements). No one realizes how much a good floor job improves the look of their space until you convince them to let you perform the service. Once it's done their hooked!
Best of luck to you!