Because I’ve sold quite a lot of work I have two or three guys who are ready to snap up any work I might be selling. You could put an ad on gumtree or Facebook, or you could approach window cleaners directly, asking if they are looking to buy any work.
I tell the customers that due to downsizing I am unable to continue with the service I provide and I thank them for their custom and I inform them that X will be taking over from me on the next clean. For those that I don’t manage to tell face to face I put an explanation letter through their door, or give the new guy the letter to put through their door/hand to them when he cleans their windows.