They're on ebay all the time... unfortuantely VERY pricey
Usually the BARE button (without cone) goes for $15 a piece
The cones range from $5-$15 each!
Whole ones $25-$30 typically in both black plastic (low or high hat) or brushed aluminum (high hat) styles.
I found nearly identical ones on RadioShack's web site, but they're no longer available by internet, phone or in stores
GRRRR And I'm NOT paying $25 a pop for them.