i was wondering if you (or anyone who comes across this comment) know of a brush similar to this one but for paint tool sai? terreverte.tumblr.com/post/903… it is rectangular shape but soft and has sort of overlay effect??
Digital artists usually use either drawing tablets like Wacom or Huion, just flat boards with pressure sensors that you use a special stylus on to transfer lines to a digital art program like Photoshop or Paint Tool SAI. Or some professional (and lucky) artists use Cintiqs, which are actually monitors you draw on with a stylus. For digital beginners, I recommend a Wacom Bamboo. You can buy later models for cheap on Amazon and Ebay just to practice with.
I'm pretty sure they're not using a mouse to draw. Most digital artists use something called a graphics tablet. The most popular company is called Wacom, so here's a link to one of their website pages to give you an idea of what graphic tablets are about: www.wacom.com/en/us/creative