The problem is usually that your model file either contains no texture references at all or contains relative paths to the textures that don't resolve from the location you're loading it from.
Try opening the .egg file using a text editor (really, you can) and taking a look at the entries, if they exist at all. Make sure that the specified path to the texture files is a path that is relative to the location of the model file, or to a different location that is on the Panda3D model-path.
As for your other question, there are a bunch of low-quality art resources for use as placeholders in some obscure corner of the Panda3D site: https://www.panda3d.org/download/noversion/art-gallery.zip