Hey guys, just thought I'd let you know that I found a version of SAI floating around that is actually transparency enabled, safe to download, and it doesn't glitch out and reject saving as a png if it comes in contact with a PSD like some versions do. I'm really excited to not have to switch to Photoshop when I want transparency, and I thought it might help a few other people on here!
Here's the download link!
By the way, if you already have SAI and decide to switch to this version, I'd suggest screenshoting all your brush settings and redoing them, so that the brushes don't feel strange to mess with, as you might have different settings than the defaults on this copy.
By the way, the way the transparency works is when you save the file as a png, it gives you an option to save it as "24bpp RGB" or "34bpp ARGB (Each pixel have transparency)". Save it as the second option, and you're good to go!