Bite the bullet and go to a proper old-fashioned department store to get measured. Rigby and Peller are the very best, if you're anywhere near London. Take her out to lunch and make a day of it.
I've taken my 10yo dd with me to R&P when I've bought bras for myself, so she knows that when her time comes that's where we'll head to. If she doesn't get over the embarrassment at the beginning, she's got a lifetime of ill-fitting bras ahead. And who needs that?
I took dd1 for a bra fitting about a month ago. We hadn't intended to, but I agreed that if she wanted to she could just wear a bra on PE days. We went to M&S and came out with 2 bras which she has worn constantly since.
Sports Bra and Lace Trimmed Bra You can also buy them off the shelf - best to see if dd will be measured though, and try some on. DD1 was measured at 28AA, but the bras were too tight around the ribs, she ended up with 32AA which she says are really comfy.
Carolina Moon -- Rigby & Peller do a whole range of makes of bra, and yes, if I'm honest, they are on the expensive side. Not pisstakingly so, but still.
But it is so, so worth it. It's a lovely boudoiry atmosphere, very girly, with lots of middle-aged women being down-to-earth about their physical imperfections. Not scary at all, but def. a treat, hence an ideal place to take a youngster for her first bra fitting. And you come away with a bra that really fits, whereas I always come away from M&S with a bra that almost nearly fits. And it's cheaper than a boob job. A bargain, really.