Generally Shimano does not sell individual cogs. You will have to either live with it or get a new cassette.
Not true at all. Shimano sells individual cogs, as does Miche.
http://harriscyclery.net/product-lis...s-93/?rb_br=53
http://harriscyclery.net/page.cfm?Pa...and=572&type=T
As well as a ton of other shops, use google.
But, seeing you can get an entire HG-30 CS for under $10, why bother?
http://www.ebikestop.com/shimano_hg3...tte-FW8485.php