Where can I buy cheap paper? If it is online that's a plus.
Paper for what? As you might understand it varies.
For papercraft, go to Staples or any office supply place to find heavier papers.
For origami, you can try the usual http://www.origami-shop.com/ (note they have a different site for the USA).
Also craft and hobby stores often have little packs of like 300 kami slices or something.
As for what paper you need as a noob, if it is for papercraft, just use regular printer paper. If it is for origami, kami and regular printer paper suffice.
https://www.walmart.com/search/?query=origami%20paper&sort=price_low