First things first. There's no reason to be ashamed. Some people may judge you and you may still be judging yourself, but there's really no need to. Love yourself for who you are and try not to let other people get you down.
It may be a little early for this suggestion, but consider trying to become comfortable enough with yourself to be comfortable reaching out to the community. In most major Western cities you'll find like minded people and there's safety in numbers. I know that that may be a big step for you, but finding a shopping buddy will help tremendously.
Many of the trans folks and cross dressing folks I've known find it much more comfortable to shop with a supportive friend and it's always nice to have a second opinion on an outfit that you've picked out. I've been a shopping buddy on a few occasions, it's great fun. Some retailers are even allowing people to join you in dressing rooms to offer a private opinion, so that you don't have to walk out in full view.
If you're not comfortable reaching out yet, you can always just tell people:
Oh, I'm shopping for someone special.
Which has the benefit of being absolutely true and maintaining your privacy.
But I strongly recommend learning to love yourself and trying to reach out. You're not alone and there's an entire community of like minded people out there waiting for you.