Just knock every door, ask politely if they have a window cleaner , as you are working in the area, if they say yes.....say ," thanks for your time, could i leave you a leaflet should you ever need me"?
If they ask for a quote , give them one, why would you feel uneasy without wfp?
Dont mention the system you use at all until they mention an awkward window, also they may say "you can leave that one as it is hard to get to"
Say ok, and when you get your wfp they will love you for cleaning that one.
If you have a problem with knocking houses with hard to reach windows , leave them until you have your gear.
be selective in that case , that will make you feel more confident on the door .