As another poster said, Muji has very affordable ones in a variety of styles (gridded, lined, plain, record). They're very thin and cheap, but I love them and have been using them for years. Alternatively if you're near a Kinokuniya (they have a couple US locations) they have a stationery section with other common brands (e.g. Campus).
If you really want to have it, you can order directly from Japan. You can use a reshipping service like http://honto.jp/ and have the company you buy from ship to them if they don't ship internationally. They also help you order things if you don't know the language.
Then it just comes down to the import rules of the US (seems like you're fine with anything below 200$ of value, but I'm not sure [1]).
Yes, it's a bit cumbersome, but in this glorious age of the internet there should be nothing (legal) that cannot be procured.
