NEW UPDATE FROM VERSION 05-19-2019, SEE LAST NOTE AT THE END tl;dr: [Free Brushes]
===========================As for the lastest update 0920 (09-20-2018), Mr.Koji Komatsu has finally added this feature into SAI, Brush. By a weird name, Scatter Brushes(see change log), but whatever work, work.
The method to create one is also doable, but requires certain workarounds.
I've been searching on the internet for some brushes, but as expected, there wasn't any new brush in new format for SAI 2.0.
So, I decided to make some myself, and I'm planning to share them with you guys.
There're lots of them, but I'll go with the most stable and disting