My heart goes out to you King Phoenix. I am much older and don't have to deal with such things but intolerance within a family tears me up inside. Many hugs.
I second Peep's comment about ordering things by mail. Double check the terms on their website but many, including most of the major ones, offer free returns and I'm planning to buy some dresses from Amazon even though I know I'll cry if they don't fit the first time (or at least are a little big so I can take them in), I don't know if I could stand waiting for a new one (fortunately, Amazon will ship the new one out before you've even returned the original, they'll just charge you twice if you don't get it back to them within 30 days).
Another option, if there's a Kohl's or a JC Penney's near you (there are probably many other options, those are two I know well), they have lots of dressing rooms in each department and no gender labels on them (and no one standing guard to judge, they rely on security devices on the clothes and cameras outside the dressing rooms to deter shoplifting). I've tried on dresses and other clothes in both places and actually never even encountered another customer face-to-face, though I think if I had the worst I would have gotten would have been some surprised glances if they saw me with dresses or lingerie in my hands. A lot of other stores have similar ungendered dressing rooms, with or without attendants. I know Burlington's has gendered (unattended) dressing rooms, I was there the other day. If I'd found anything to try on, I might have used the men's or maybe just gone into the women's with my wife. It really doesn't make any difference to me and I doubt it makes a difference to anyone there. It's not like using a locker room where everyone can see each other while their changing, or even a public restroom, at least that's how I feel - but then I don't have your problem with an overbearing parent.
I hope you can find some useful thoughts in this but I mostly want to tell you how much I want to give you a great big hug and tell you how awful it is that you have to deal with this.
Love,
DeeDee