Why don't you buy them online ? See here for example :
NGK spark plug BKR6-EQUP (plugs) BKR6EQUP platinum - eBay Mobile (item 270596874222 18/11/2010 16:16:29)
Much cheaper as you can see. I know that this particular eBay seller is UK based (I bought 8 plugs there recently) but there's got to be some US based online sellers.