For those that want the official Silly Putty, you can by it in bulk from Crayola:

http://www.crayola.com/store/sp_index.cfm?dc_cid=194

For those who do not know, you can buy it by the name:

"Dow Corning Dilatant Compound 3179"

This in even larger quantities.

There are more than just one color though.  Here is a site that is worth checking out for, tricks, pictures and even purchases, an interesting site!

http://puttyworld.com/