Best place for a first bra?

[Cabbagesandcustard]

DD is 13 and finally and reluctantly concedes that she needs more support than a crop top can give her. She's very petite in every other way so I'm looking at the smallest back sizes.
The local department store where I was fitted for my first bra, as was my mum (quite a family tradition!) has only one [ugly] bra in her size - they suggested trying M&S for more choice. However, there seems to be very little in the M&S Angel range that is plain unpatterned white (patterns show through school shirts) and not padded in some way (I can bear this!). She's also too old for all the pink which seems to feature heavily in first bra ranges.
Does anyone have any better suggestions of where we can try?

I've just had a look at the M&S website and there are a couple of styles that might be suitable - I was thinking of a fitting but perhaps we will have to order online - why oh why do they have such a poor selection in every store I've looked in? They really need to sort their stock policy out.
Anyway... other suggestions still welcome.

John Lewis?

Ask in Style & beauty topic too, but personally I'd avoid letting M&S do the fitting as they are notorious for using an outdated sizing method, and usually try to put people in bras that are too big in the back and too small in the cup.
You can measure your DD yourself, or get her to measure for a starting point. Measure tight around the ribcage (that number or the ones either side if odd are the back size).
Looser around the fullest part of the breasts to get cup size starting point.
Take the difference between the two measurements:
1in difference is A
2B
3C
4D
5DD
6E
7F
8FF
9G
20GG

My guess, if she's slim as you say, is that she's about a 24DD or 26D.
Measure and report back
(I love a good bra intervention)