where can i buy a spring loaded, push-button type killswitch like Buckethead has?
Radioshack. Get a momentary-off switch.
Radioshack, or The Source by Circuit City if you live in Canada. It shouldn't run you more than 4 bucks.
https://www.ultimate-guitar.com/forum/showthread.php?t=315275 There's lots of good links to electronics supply sites in here.