commercial work is out there, but you have to look hard to find it.
As often or not, when they have found a reliable window cleaner they will stick with him, I've found them far more reliable as customers than domestics.
Keep your eye open in and around your local town, there are always new businesses renting offices and so on, the trick is to catch them early.
Other offices, every now and again you'll see the signs up showing offices for rent, keep your eye on them, eventually someone will move in.
check out the various offices and so on around you and see what state their windows are in, you'll be able to tell which ones are having their windows cleaned, call on those that aren't.
It isn't easy to collect laods of commercial work...takes years really.
Ian