@irontoenail - Another source is to just make paper yourself. I don't mean necessarily from scratch, although that's definitely an option if you have the time and resources. I know people who make paper with leaves and dried flowers and all kinds of things embedded and it can look amazing if it's done the right way.
But even if you don't want to make the actual paper, you can dye it or otherwise change it in order to make it unique.
Dip dying in particular is really easy and effective. It's where you put an oil based dye or paint into water and swirl it into patterns then dip the paper into it to transfer the colors onto the paper.
No two sheets will be the same and it's usually light enough that you can still use the paper for almost any purpose, including things like origami (which can be difficult to manage with recycled or homemade paper as it tends to be too thick).