Vintage plus-sized clothes are hard to find. In general, American women are taller and heavier than they were in the past. American women weigh fifteen pounds more than they did twenty years ago with 68% wearing size 14 or above. While the average height was 5'3" and weight 140 pounds in the 1960s, today American women average 5'4" tall and 168 pounds.
When I was a skinny young thing, I had trouble finding vintage clothing that fit properly due to being tall and somewhat broad-shouldered., so you don't have to be a plus-size to have trouble finding older garments that fit properly.
Some vintage stores are trying to source larger sized vintage clothing due to demand. There are some online sites like Ballyhoo Vintage Clothing which offers plus-sized vintage dress from the 1930s through the 1960s. Etsy offers larger sized vintage clothing, especially garments from the 1970s.
Berriez, a vintage shop in Brooklyn, New York sells plus size vintage on its Instagram account. Also check out Belen's Linens, Blue Velvet Vintage, and Bloomers and Frocks.
You can also look for retro or vintage style, new clothing made to look like actual vintage. If you are looking for the real thing, authentic old clothing, be careful when you shop online. Many sites make it look like the clothing is actually old when it is a reproduction or based on vintage clothing.