I found this from CNN trivially by looking up on google. Sore-Loserman: From political parody to charity's windfall, From december 2004:
When printing company owner John Kanis jokingly came up with his "Sore-Loserman" takeoff on the Gore-Lieberman campaign bumper stickers, he had no idea how well his idea might stick with the voting public.... "We originally did these for fun," said Kanis of the stickers, which first were handed out free. But when the first supply ran out, the phone began ringing. And the stickers have only become more popular since the still-undecided election was held November 7
It seems clear, unless CNN was lying, that some guy named Kanis invented this, checked with a lawyer, and started printing them all before the november 7 election date.
Is there another story that is at odds with this one? I wasn't here until 2005.
This was discussed on FreeRepublic at the time:
http://www.freerepublic.com/forum/a3a2ce4915704.htm
Post 125 gives a good overview.