I wasn't able to find, on a casual search, any images that would be too small to fill a 3x5 notebook.
Don't be misled by any "72 ppi" that you see. It's a very common misconception (even among beginning designers) that this is something to pay attention to. It's not relevant at all for print purposes, because all that matters is the actual resolution (in pixels, not pixels per inch) of the image. (The confusion enters in because of an unfortunate use of terminology: the same word, "resolution," is used to refer to both the absolute number of pixels in an image and "pixels per inch." In this case, only the absolute number of pixels is useful.)
For a 3x5 page background image that's at 30% or less opacity, you don't really need more than 450 x 750 pixels, which works out to 150 ppi, if you're printing with a digital press. For offset, 200 ppi (600x1000 pixels) would be adequate. Both of these are very common numbers for those enlarge "ghost" images that fill a page on which a smaller, full color version is displayed. It's a popular design style used in pictorial calendars and photo books.
In the case of these particular images, especially the photographs (which are almost all soft to begin with), you could get away with less, but the point is moot because all of them are more than large enough for your purposes. Just be sure to download the "Original" resolution and you're good to go.