For me, materials are one of the most diificult aspects of 3D. Blender provides a great interface to creating and manipulating materials & textures, but it's the content [finding good textures] that really slows me down. Recently I've been rendering a few planets and had to track down some image maps. Here's what I've found so far.
The first place you'll want to check out is, of course, NASA. They've provided a series of images maps for objects in our Solar System as part of the Solar System Simulator. All the planets and a few of the moons are included.

Bjorn Jonsson has a planetary maps page that has few nice maps. Along with the maps Bjorn site includes planetary rendering tips and experimental renderings. It's also worth checking out his models page, which has some great POVRay renders of the Galileo & Cassini spacecraft, and a link back to NASA for some free models.
James Hastings-Trew's Planetary Image Maps site is also a good place to go. This is the first site I've come across that has image maps of the Sun. James also provides example rendering and comments for some of the images.
Above, you'll find links to pages where you can obtain high-quality image maps of the planets for the purpose of 3D rendering. You'll find notes on the creation or source of each map, and a small sample rendering of the map to show you how it will look in your artwork. Interspersed amongst the pages you will find the occasional rendering tip.
The Nine Planets [A Multimedia Tour of the Solar System] also has a planets picture appendix with links to all the objects in our Solar System. If you can't find a good map from one of the sites mentioned above, you're bound to find a picture of it here. It may not be mappable to a sphere, but you can always do a bit of tweaking yourself.
If you know of any other sites containing planetary image maps, please leave a comment and let me know.