The LASER FORCE line is in-scale...
http://www.virtualtoychest.com/l/laserforce/laserforce.html...I made my first Star Wars and GI Joe customs with these around 95-96.
They can get pricey if in good shape nowadays... but for fodder you'd want some junky ones.
Also, some other fodder to think of...
Lanard's STAR FORCE are cheap and plentiful especially in the UK & EU.
Also in scale.
here's one of the more notable pieces...
http://cgi.ebay.com/Lanard-Star-Force-Space-Shuttle-Launch-play-set-/280692426856?pt=LH_DefaultDomain_0&hash=item415a928868...they also made a lunar lander and space capsule which i've always wanted to customize.
I've had a huge amount of luck hitting the discount bins at my local model shop, getting tons of kits for little parts and pieces for cheap. I also got 50-75% off for cracked or damaged styrene, which has saved me some cash too, all I did was ask at the counter and the guy gave me an amazing deal!
I've also been buying any and all of the recent movie-GI JOE vehicles I find for cheap as they can make good starting points for customs...
I'll add some more when I get home tonight...