Get one of the supalite brushes from Gardinerpoles, cheap & cheerful and they just happen to be a very good brush.
They are Bentley brushes, and I hear that the likes os Asda and Tesco sell them.
Just a dirt cheap little sweeping brush...who'd have thought they'd turn out to make a top WFP brush

I suppose you could buy the brush for a couple of quid, get the jets and bits & pieces for a couple more quid and make the conversion yourself, but to buy the made up brush will only cost you £15 anyway, hardly worth the caddle of spending time converting one.
Having siad that, I had just parked the van and glanced out the window and saw a dumped brush head...."I recognise that shape!" methinks :

I pluck it out of the mud and see it is indeed a Bentley brush, perfect nick...(er...if a little dirty

)
It's been chucked because the screw part of the handle had snapped off inside the brush itself...
Anyways, I'll probably buy some jets and so on and convert it as yet another spare brush.
They are not really monofilament bristles, but once wet and in use, to all intends and purposes they are so.
Vikan have also got a new brush, not as light as the Bentley one, but it looks really good, it's square now and not rounded off, monofilament bristles too, so Ideal for WFP.
Just a little more expensive than the Bentley one, well, a fair bit more expensive, but it still isn't particulary expensive in real terms.
Ian