Try asking local window cleaners if theyve got any work to sell, check in local papers dont pay more than 2 or 3 cleans for residential comercial is different it will be priced a lot higher.
I am just negociating some comercial myself looks like its going to be expensive for me
Also make sure the work is compact and established i dont do anything under 10 pound so you you want to earning between 20 and 30 pound per hour. I can easily do 150 a day 200 if i push it on a full day but if your just starting out do not try and hit these targets straight away take your time and do a good job.
best of luck
Brett