A sock with the end cut off & 2 leg holes works well. There's a craft section on here for ideas for small chis (no such thing as a "Teacup"). There are tons of websites including ebay that sell dog clothes.
Is this for your new 6 week old puppy? If so I would hold off buying anything for as long as you can because he will grow so much before he needs a winter coat. It's tempting to go crazy and get them a million things when they're little I know. I got my first 2 in summer so they didn't need anything but my new boy just arrived and it's mid winter here so he just has one thick fleece vest that he wears almost constantly. I've only had him for a week and a half and I think he will only get 2 more weeks out of it and it will be too tight! For later on try Hip Doggie, I'm really happy with the quality of Winnie's vest that I got from there and he can't slip out of it like most other brands of dog clothing I've tried.