Millions of uninsured Americans who missed the sign-up period -- perhaps hoping to shop for coverage at their own convenience -- may soon find they can no longer buy comprehensive coverage whenever they want. "If somebody needs insurance in the middle of the year, in some places, they won't have access to it," said Kathryn Gaglione, a spokeswoman for the National Association of Health Underwriters in Washington, D.C., which represents health insurance agents and brokers across the nation.