Complex sentence: There is record of these flocks surviving until 1916, and also record of another flock in Illinois in 1917, but by 1921, Shetlands could not be obtained in the U.S. (The Sheep Breeder, 1921).
Split into simpler sentences: There is record of these flocks surviving until 1916, and also record of another flock in Illinois in 1917. By 1921, it was recorded that Shetlands could not be bought in the U.S. (The Sheep Breeder, 1921).