Finally!

I have been searching the internets for years and I found it, a Towelie towel! (Affiliate link)

From Wikipedia:
Trey Parker and Matt Stone explained on the DVD commentary for “Towelie” that the character was conceived as a joking reference to the over-marketing of characters in the wake of the series’ success. Eric Cartman delivers the penultimate line of Towelie’s eponymous episode: “You’re the worst character ever, Towelie,” to which Towelie responds, “I know”.