Commercial window cleaning rounds are absolute gold dust, the very best of luck in trying to find one.
I have one myself and I could take my pick of who I would sell it too!
It has taken 20 years to build it up.
If you cannot find one, my advise would be to canvas every shop and office in your local major town.
The pickings will be slim, but you need to get a toe hold.
Don't go in cheap and undercut, you will be the loser in the end if you do so and will creat animosity this the local pro's.
Once you have a couple of shops/offices, make sure you are there whenever a new business starts up, all the other window cleaners in your town will be after them too, no such thing as 'My Patch', there all always at least 3 or 4 pro outfits operating in any town, and the bigger the town, the mre there will be!
If you are lucky enough to be able to be able to buy a commercial round then more power to your elbow! But be prepared to pay vastly more than you would for a domestic round.
By comparison, commercial customers will not change their cleaner willy nilly compared to domestic, if they have someone who is offering them a regular, consistant and quality service, they will stay with them.
But the very best of luck to you.
Ian.