I had the same problem with clothing last year when I first got Jeter in December. I had to make him sock sweaters. I used all different kinds, you know the fuzzy ones and the Argyle ones, they look so cute when made into a sweater! Jeter is still tiny at one years old, his weight is 2 pounds 8 ounces, and I find it hard to get clothing to fit him still, its so loose on him he falls out of the clothing! He still sports the sock sweaters now that it's chilly again! Some places do carry xxxs clothing if you are lucky to find it, they fit a lot better than the xxs size.
Just seen the photo of the sweater you did with the sock, Missy is so so cute!!