Besides being included in the Celestia distribution, the official texture repository is on SourceForge at
http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/celestia/celestia/textures/
There are problems with the textures which will be fixed in v1.2.5 but haven't been uploaded to SourceForge yet. In the meantime, a corrected set of textures and solarsys.ssc are available at
http://www.lns.cornell.edu/~seb/celestia/hutchison/
Specifically, the current textures don't all have the same offsets. Some have 0 longitude at the edge and some have it in the center where it's supposed to be. Also, some of the planetary and satellite rotational offsets in Celestia (in the file /data/solarsys.ssc) are incorrect to a greater or lesser extent. As a result, the images of many bodies have the wrong orientations. Mars has the wrong side facing Earth at opposition, for example.
Some very high resolution textures for Celestia are available at
http://www.shatters.net/~t00fri/texfoundry.html and various other sites on the Web.
I hope this helps a little.