I have no idea how brushes in Sai work -- never used it. Assuming they work the same way as others do, though, you can download my image pack (link in the description) and make them into Sai brushes. As long as Sai uses white as a transparent background in its brush templates (like Photoshop, GIMP, and others), they're ready to be made into brushes pretty quickly.
Yeah, it's not an ABR pack that I'm talking about when I say to go to my website and download the image pack. They're JPGs, but those are easily converted to BMPs. If it uses the same white=transparent/black=opaque method that almost all of the other programs out there seem to use, then these would be able to be used to make Sai brushes. Even if they're not, you can download the JPGs and find a usage for them, I'm sure. (There will ALWAYS be a way to make the JPGs into brushes, even if it means using them as a mask or some such, but I don't know enough about the program to walk you through it)
Not sure, never used Sai. And these weren't made for it. But you can go to the link in the description for the image pack, download that, and look up a tutorial online for how to make brushes in Sai. The JPGs you'll get in the image pack should be ready to be made into Sai brushes.