Boxer don't really grow a *winter* coat. They'll get a bit more hair, but since it's virtually all guard hairs, it's not insulating. If you look, you'll see that they don't have an under coat (a winter dog has 2 fur types- guard hairs and a soft fluffier undercoat to trap heat.)
I buy interesting coats on sale, and also make my own. You can buy dog coat patterns or make one that is just like a horse blanket only mini size. You measure from collar to tail, then center chest around the side to point of butt, then how deep you want the coat to be. Stitch it together at the chest, add a velcro belly band and you're done.
As for fitting store brands: I find most sweaters don't fit boxers well, being too tight in the chest and putting pressure under the arm pits. To get one that fits the chest, you end up with one too big in the waist (which you can take in with a tuck on the sewing machine.) Same for Jackets.
I can never keep booties on the dogs. They have small tight feet and the booties just fly right off. When mine get cold feet, they start holding them up; then it's time to go inside (although they are cold, they usually want to stay out still. ):p