Where did Dolores Umbridge get her evil quill?
I would doubt she bought her evil quill at Scrivenshaft's Quill Shop in Hogsmeade (If Scrivenshaft's were in Knockturn Alley I'd have a different opinion.). Would this kind of item be issued by the Ministry of Magic? Or did she buy it on the black market?
Umbridge is known to buy items from peddlers on the streets (In the case of Mundungus Fletcher, she actually stole the Slytherin locket Horcrux by threatening Dung with arrest for not having a proper sellers license.).
A quill like that seems like an ... odd ... thing to just keep on hand, so she clearly went out of her way to procure it, I'm guessing especially for Hogwarts. So, Knockturn Alley? A Dark Arts dealer? The Ministry of Magic? A design of her own making? Is there any canon information on where Umbridge got her I must not tell lies quill? I went through Harry Potter and the Order of the Phoenix and didn't find anything, but there are six other books.
‘Now, you are going to be doing some lines for me, Mr Potter. No, not with your quill,’ she added, as Harry bent down to open his bag. ‘You’re going to be using a rather special one of mine. Here you are.’
She handed him a long, thin black quill with an unusually sharp point.
‘I want you to write, I must not tell lies,’ she told him softly.
Order of the Phoenix - page 240 - Bloomsbury - chapter 13, Detention With Dolores
UPDATE: November 2014 -- J.K. Rowling revealed on Pottermore that Umbridge indeed created her infamous evil quill all on her own, so all three answers below are canonically correct!