I’m was that size at that age so understand!
Debenhams to a fab range, and if you look on eBay you will find many of last seasons at a decent price
Avoid M&S to be measured/ at 16 they told me I was a 32B, I went in wearing a 32DD! I queried and they said they were right, went and got me bras to try I, and I burst out laughing at the sales women’s face when I pulled back the curtain and my nipples were hardly covered and I was spilling out from the sides and back. Never went back again!
Bramissivo- if you have one near, or order a few sizes and send back
Figleaf- as above
John Lewis- do some nice ones, but can be pricey
New Look- I think does her size, of it’s the right size she is wearing
ASDA- the large one near me used to do up to a G cup in lovely bras, ones with cute patterns, underwired, and I would say really teen friendly. I haven’t been in a while so don’t know if they still do, but they don’t do them on line unfortunately! (Typical!)
I can’t think of anywhere else. But once you know her size, eBay and Amazon are good to look for larger sizes at a decent price. I’m a GG and I buy a new matching set every other month, and pay about £25 in total.
Hope you find some!!