I like to buy Singapore books from www.bestedusource.com, but if I have to put in an order for other things I will buy them from www.christianbook.com or www.rainbowresource.com.
If you're just going to try it, I would only buy level 1A materials. Workbook, Textbook and Home Instructor's Guide (HIG). A lot of people think you don't need the HIG, but if you've never taught Singapore before I think it's essential. First of all, it has all the answers. :) Secondly, it helps you understand the singapore method. I've heard a lot of moms say that it didn't help their kids or they didn't understand it, but most of them weren't using the HIG. It includes some games for reinforcement and it keeps you on track with facts memorization. (We use Calculadder Drills for that.)