BenI usually get my paper from art stores (Michaels, AC Moore, etc.) But as for online sites you can try Amazon and eBay.
Happy Folding gives you a list of online origami stores.(some are outside of the U.S. but can probably ship to your location).
For origami diagrams, youtube is a great way to start learning it (keep in mind they do a variety of origami not just kusudama/modular type)
That’s all I can think up now, but if you look at other origami artists on tumblr they usually put a link to the diagrams when they post up pictures (as well as I do but not for the older posts).
I hope that helps!