V'LOCKer toys might be a source for interesting parts:
http://www.volks.co.jp/jp/vlockers/wepn04.html/
http://www.volks.co.jp/jp/vlockers/wepn08.html/
(actually these re interesting -- in that individual parts are relatively generic -- it's how you combine them changes the character. For example a row of "cogwheel" parts becomes "chainsaw". A cluster of tubes connected by "drive wheels" into cluster turns into "gatling", a stack of wheels mounted on shaft with "turbine" at one end and "exhaust" becomes "jet engine", etc.)
perhaps you can find stuff out of Zoids toys?
I do vaguely recall some motorized vehicles (sort of like the Tamiya mini-4WD cars, but they were SF themed combat vehicles), for which there were various "weapon" and "accessory" parts (varying from things that look like some blaster guns, gatling guns, drill bits, etc, as well as things that looked like jet exhausts, turbine wheels, etc.) -- don't remember the manufacturer -- it was more than a decade ago (though Aoshima comes to mind).
Wave has the Option System parts which could be used for various things
http://www.hobby-wave.com/LINE_UP/optio ... index.html
I also recall there's a Korean manufacturer that sells similar items - typically targeted to the Gunpla (Gundam plastic model) audience, but would be usable for general SF type projects.
also various "found items" or "recycled items" could be useful -- including stuff like toothpaste caps ("jet nozzle"), ballpoint pen parts, etc. Parts from broken printers, CD/DVD-ROM drives, etc could yield usable parts. Key point here is to combine the parts in various ways. L'Eggs pantyhose used to come in largish egg-shaped containers have been used in various studio models (e.g. the domes on the front of the nacelles on the Y-wing). Similarly plastic Easter egg capsule could be used for "rocket engine bell" -- just add some strips to make it look like the plumbing regenerative cooling and fuel lines.
Relatively simple shapes combined in various ways could yield interesting effect -- take a look at these model rockets.
http://fliskits.com/products/rocketkits ... decim8.htm
http://fliskits.com/products/rocketkits ... /tesla.htm