As taylor mentioned, looking for 00P bottoms would be the way to go. Anthropologie, White House Black Market, Madewell, and J. Crew also carry 00P bottoms. I think Madewell and J. Crew even occasionally carry 000P bottoms (size 23).
Back several years ago, when I was 87 pounds (I am 5'2"), I tried on some Maje bottoms at Nordstrom, and the smallest size was too small for me. So if you are looking for a designer brand, you could try Maje.
American Eagle runs small, in my experience, and would be a good place to try for a cheaper option.
In my experience, Macy's bottoms often start at 2P, sometimes 0P, so they might be too big. But the juniors' section might have skinnier offerings; juniors' bottoms are cut for a straighter figure, I think.
Topshop runs 1-2 sizes small and would be another place to look. They are sold at Nordstrom.