Autoglym do some pretty good stuff for the foam cannons, their Polar Blast seal and wash range is really good.
There is one called Snow Foam from PRO KLEEN which works well.
I've also tried and quite like the Meguiars Ultimate car snow foam kit, it's pricey but built to last and works really well.
You can actually get a lot of these items at no extra costs on our Amazon link and the sales will earn us a small commission which helps to offset our running costs.
i use the autobrite direct foam cannon which is spot on but theres a whole array of them on the market and id imagine any of them, from the main brands should do the same job.. avoid the karcher one though absolute waste of time