I offer a few ideas to buy origami paper:
In bookstores you can find a lot of not specific origami paper great for folding. Apart from normal sheets of 90g, white or colored, they also sell wrapping paper (kraft paper). This kind is great when you need large and thin paper. In some bookstores also sell one of my favorite paper, the Tassotti. The sheets have a size of 50x70cm or 70x100cm. Is a bit more expensive than regular paper but looks great in lamps, Bascetta stars… so worth it.
The Danish company Tiger often shows in their stores several kinds of papers. My experience with them has been very diverse. Long ago I acquired a package of 100 or 200 colored papers for 2 euros which broke when fold. Very cheap but not recommended. However, recently I was given a package with 40 Tiger origami papers decorated with diagonal stripes that are perfect.
This summer, walking in Madrid, I discovered the Muji store. Here you can buy a package of 80 paper kami face with 27 different colors for just 2 euros. If you like Japanese decor papers they also sell packages of 40 sheets of chiyogami for 5 euros. It is far more expensive than the first option but the texture of these papers is amazing.
For me, the best shop online is eBay and for that reason can not miss this list. If you type the words paper and origami you’ll find thousands of results. It requires some patience to find exactly what you want but there are precious and cheap papers. The problem is that you can’t control the quality of the paper through the screen, so every purchase is an adventure. However most of my experiences have been positive, so I encourage you to experiment with online shopping.