En designm.ag They have left us a very good post with 30 images with textures of cardboard ready to download.
The average size of the images is 15x10 cm. and they have 72dpi, enough features for us to use these images as much as for textures in any montage, as well as to take a sample and add it to the texture bank of any of the design programs that we normally use.
Download | 30 cardboard textures