I've seen it everywhere from a craft store like AC Moores to Office Depot to Walmart. Even sometimes at the drugstore. Make sure it says washi or paper on it. There is duct tape that comes in little roles and looks almost exactly the same until you open it. They also have decorative scotch tape that look similar but isn't.
It is very different from duct tape. Washi tape is also called paper tape (I believe "washi" means "paper" in Japanese but don't hold me to that). It comes in all kinds of patterns and colors and is often translucent. It feels kind of like masking tape but a little thinner. One nice quality is it can be repositioned. So if you use it on paper you can pull it up and move it without it ripping the paper. You can use it on walls or other items without it being permanent b/c it doesn't pull the paint off when removed.