The blocks don't need to be exactly 1 inch. They're intended to be the small kind you often find in a child's home—so they should be about 1 inch in size but certainly do not need to be exactly that size. Basically, you want to have small blocks available that a child can easily grip and hold in one hand and that can be easily stacked.
Since ASQ-3 is designed to be flexible enough to be completed at home by parents, lots of different kinds of blocks would work. Some toy stores won't have the certificates about lead paint readily available, but educational toy suppliers online usually do. These Melissa & Doug alphabet blocks would work fine.