OK so I know a lot of people have their own recipes for making brush cleanser, but I've been testing this simple recipe out for 5-6 months now and it's serving me well so I thought I'd share. I pretty much use this every day to sanitise & clean my brushes - I can clean them all quickly and they're ready to use again in a few minutes.
It's literally just 1 part spray-on leave-in conditioner, to approximately 7 parts isopropyl alcohol. The isopropyl alcohol is the basic ingredient in most brush cleansers, as it klls germs and dries in minutes. Unfortunately it can also be somewhat drying to natural fibres, hence the addition of the leave-in conditioner - it keeps the brush bristles nice and soft. You don't want to go too overboard on the conditioner in your formula though, otherwise you get residue in the brush and they take too long to dry.
Isopropyl alcohol is dirt cheap & can be bought in pharmacies (I used to get odd looks at the chemist in New Zealand, as I'd buy 4-5 bottles at a time! I swear they thought I was making drugs...) or from Maplin in the UK.
For the leave-in conditioner I'm using a cheapie one from Boots. Choose one that has a consistency like water, since a creamy consistency won't mix well with the alcohol.
Mix and pour into a spray bottle. To use, spray the brushes with the solution & wipe clean with a tissue. This is meant to be a spot cleaner only, so you should still deep clean your brushes regularly in water.