Struggling to get a first bra for teen DD

[CarrotCakeMuffins]

My teen DD has been wearing cropped tops since starting secondary and has now said she would like a bra.
But due to Coronavirus I'm unable to get her measured anywhere and she'll be unable to try anything on while out shopping as changing rooms are closed.
So, where are the best places to get first bras in small sizes? And what size(s) should I be looking for?
I've measured her at 28.5" band and 31" bust. So I thought 28 B?

I ordered a selection from M&S which are all tiny bands and way too tight for her (the band itself is only 21" un-stretched!). So they obviously still do silly sizing. These will all be going back...

We are near a small city so have a reasonable range of options available, and am also happy to order online, and return as necessary.
I don't want to go in more shops than I need to at the moment...

Where should I be looking for small first bras for teenagers?
And how does sizing work in those places?

Any advice greatly appreciated.

Oh yes. It was a revelation to me when I moved from 32A to 28d (in my 40s!).

I actually understood the purpose of wearing a bra.

My 13 year old is still in cropped tops, but it's very useful to know that there are plenty of cheap and cheerful options out there.

Punk by Victoria's Secret do really cool cropped tops

Don't go to m and s. Bras are shite.
Do not add 4/5.
30" back b cup. Debenhams , brastop, next etc do free returns buy a few and return ones that are no good.

The Lounge bra in Boux Avenue is suitable for a young teen. Mine had 28C at 11yo and it was a good fit.

Boux Avenue do D-K sizes so too big.

Sorry, should say Bra stop to D-K...

Are you thinking of bravissimo?
Boux avenue does smaller sizes
lounge bra