There are many model kits to buy, but why pay when so much good stuff is free. Unless the models are exceptional, only free sources are listed here.
3D Papercraft http://bj.canon.co.jp/english/3D-papercraft/nature/
A nice collections of print, cutout, and make models.
Quite a long link list for all types of paper models and toys. FREE!!!
Direct from NASA.
Solar System Exploration http://sse.jpl.nasa.gov/kids/papermodels.cfm
More from NASA
Some very nice models from the Netherlands: http://www.lansbergen.net/eng/index.htm
Specific spacecraft models - Click here