mostly kids cloths, or pending on the brand and the seller XXXS -XXS and if you are lucky XS in women's petite. one size up for the top. example this tops smallest size is 34-35 inc and your doll is 26inchs. This kids top in size 7 is 26inchs which is the same as your doll. This top (petit) is a size 0-2 (xs) and is 26-27inchs
if you have other issues, US sizes normally run large. just remember to check the bust, waist and hips. and both sets of numbers inchs and cm. Phoebe's 3 numbers Bust: 66cm / 26in Waist: 45cm / 18in and Hip: 75cm / 30in
TL,DR: Kids sizes are best, but extra small in women's petite should also fit.