I've lost a few myself. Once that little guy is gone, you've rendered your tripod quick release plate pretty much useless. You can order these replacement screws through some of the big retailers for over $10 dollars, or you can check out eBay for some cheaper prices. Problem is you've got to wait weeks for that over seas shipping, so might be something small to keep handy. Call it Preventative Maintenance..
[Update] This one would work out as well, but doesn't look like the standard 'slotted' screw head, where a quarter comes in handy. http://www.bhphotovideo.com/c/product/302185-REG/Wimberley_SW_100_Extra_Screw_1_4_20.html