Here is a nice brush by RazoRock for only $18.00. It has a 24mm knot. I have these and very nice to use.
I’ll third the Stirling 2-band. Really pleased with mine; great looker and performer for the money.For a decent and inexpensive synthetic brush, I would also recommend the Stirling brush that @Vulcan Bubbles pictured in post #6 of this thread. It was my first synth & I still use it often.
I’ve had 4 PAA brushes and gifted or sold all but the Rocket.Phoenix Artisan Accoutrements gets a lot of good reviews. I know some people who have their brushes and swear by them. I'm tempted by the Atomic Rocket with the larger handle, myself.
+1but after trying a Muhle STF (unfortunately a much more expensive brush) I have come to appreciate how well a high end brush can perform