Not sure what you mean by self-adjustable -- that you can alter it by yourself? Is it just a bit loose, or is it so much too big that it sags or obviously wrinkles? Is it a one-piece dress, or a separate skirt or pants plus top?
) is so prone to expanding and contracting week by week. It would be very difficult to fit it properly on your own, and as you say, professional dressmaking is expensive, considering the skill needed to alter a close-fitting athletic garment in stretch fabric. And once it's been taken in, with probably the excess fabric trimmed away, it can't be expanded again if your own waist expands a bit by the time of your show. Then you risk being unable to breathe properly, losing flexibility in your body, and having the seams rip during your performance -- or all three.
You'd have to weigh the cost of alteration by a dressmaker vs the cost of buying another costume at the last minute if your own alterations proved to be too tight after all. Whoever does it, don't just stand still while it's still in the basted, untrimmed stage. Move in all directions, twist, move suddenly, pull your arms in at shoulder height quickly and snap them out, fling your arms up and back and around.
In my alternate world of doll collecting, there are doll costumers who use AI to make little OOAK (One Of A Kind) outfits that way, from lightweight fabrics. At least, the costumer I know best makes the tiny shoes, purses, and other accessories that way, and cuts/stamps out the pattern pieces for the dress, which she then sews up by hand. I'm sure it's easier to do when your little fashionista is only 12" tall, though, but someone will figure out how to make the process people-size.
