the best place is your local motor factors!!!!
If you take a look at these sill brushes that VIKAN are producing, they are identical to those extendable brushes that motor factors sell for washing vans, caravans lorrys ect..
I paid £8 for mine, bought it to do consevertoiry roof's before I turned to WFP.
£8 for a brush on a 4ft extending pole and its identical to a ' Sill ' brush. even got same thread. So easy to convert, pull out a few bristle's and drill and fit some jets and Bob's your uncle, Dick's youre downfall and Fannys your aunt!
Job's a carrot!!
Goochie.