Bite the bullet and go door-knocking, you won't regret it.
Don't take any notice of these "no canassers" signs these are aimed at the double glazing and energy suppliers, window cleaners seem to be excempt from this.
You only need to do a few hours a week and you will soon have loads of new customers. If you still feel that you cannot do this then get someone to do it for you, a piece of advice, don't let them quote, just ask them to get you the leads, you follow up and close the deal, that way you don't get shafted with under priced work and over sell on what you do.