I found a deal on ebay for 4 kill switches for $1.99 + shipping. When the shipment arrived, I got 4 packages with 4 kill switches/package. That's 14 extra kill switches!
So if you want a free kill switch, PM me and I'll ship you one when you send me a SASE (padded). If you have 2 or more cars, send me a buck or two in addition, and we'll be good.
These are Radio Shack part 275-1548C Mini Momentary Push Button Switches, SPST, Contacts rated 0.5 A at 125VAC/0.25 A at 250VAC. Normally closed. It looks like they'll fit in a 1/4" drilled hole, and the red or black buttons are about 3-16" diameter.
Standard liability disclaimer applies. I'm not liable for what you do with your switch. Be aware there are complaints about poor contact with an earlier version of this switch. All of the ones I've checked with my multimeter seem to work fine. I plan to check every one before shipping them.