I usually measure 28F, though in some styles 30E fits well. It is almost impossible to buy bras in 28 (unless you want 28AA for young teens) and even 30 isn't easy. John Lewis used to do good t-shirt bras in 28F but seem to have stopped. All the usual suspects (M&S, Debenhams, etc) don't stock 28 and hardly stock 30.
Any suggestions?