Many come from costume shops, such as mascot.costumes, who are selling many Muppet, Sesame, and Henson costume knockoffs along side a plethora of other famous characters through their ebay store.
From Sesame Street they have:
From The Muppet Show:
And from Sid the Science Kid we have:
(Sid the Science Kid)
And that last one, wow. It really must have taken them quite a long time to think up that name. I mean it's not like he's name is the title of the show he's from or anything.
Obviously there's extra irony in watermarking photos of bootleg merchandise while not even knowing all of the character's names they're ripping off.
What do you think of these knock off muppets costumes?