The ways the conversation can evolve are endless. They may tell you of a specialty store they know of, that carries heels up to size 15 or so. If this happens, good for you. This new place can serve as your sissy oasis in a desert of masculinity, offering you femininity at the price of a pair of stilettos.
If they do have your size, you will try some on. After you’ve tried a couple of pairs, you’ll try a few more. Then, you will try some more. Don’t stop asking to try on heels until you find the perfect pair.
I don’t fucking care if they’re comfortable. They just need to be the right size, stay attached to your feet tightly when you walk, and most importantly, they need to accentuate your body and highlight your dress.