I got a (dog) holiday sweater for Pig at Target. He's like 10 lbs and I think it was either a small or a medium. He wasn't super appreciative. My other cat just goes limp when you put something like that on and makes little moaning noises.
My friend has a Sphynx cat and has all sorts of clothes for him. She just buys small dog clothes. She's been putting clothes on him since he was very young, so he doesn't really seem to mind wearing them, and of course, he gets cold very easily.
She keeps a tumblr of pictures of the cat wearing all sorts of weird stuff. http://touperduper.tumblr.com for anyone that wants to see a cat wearing a Spiderman outfit.
Founder of the "I met a COTHer in a foreign country" clique!
I bought a little sweater for my kitty for when I would take him outside on the leash during the winter. He didn't like it at all. Went limp and fell over to the side. Our dog came up and pulled it off of him and then ran around the yard in triumph with the sweater in his mouth.