Singapore Math for Kids in USA

Hi friends

I would like to buy a Singapore Math books. My son is in Kindergarten and learning Addition and Subtraction. The stores are out of those books. On ebay or amazon it seems less expensive. But there are so many of them in series like 1A or 1b.. standard or primary etc etc. I can't decide which book does he wants. I tried to go on their website its even more confusing. It doesn't show the curriculum. Please help. Thank you.