I'm sorry- you hand your $100 bills (or any bills, for that matter) out to your spectators?
I would suggest taking an image (like
snibbor posted) and editing the serial number. Just type in like A1234B1234. Or type in an anagram. Something that's obviously fake to someone who looks at a lot of bills, like a store clerk, etc.
While printing out the bills, you'll want to use a double stock paper, as bills are thicker than normal printer paper.
Here's the back image:
click here
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Of course, you could always get arrested for counterfeiting bills, if you tried spending it.
I'd suggest just using twenties.