The CMS website has two sections that provide Medicare managed care enrollment information that can be used to calculate penetration rates depending on the year(s) of interest.
2006 to Current
For the latest data available, the Medicare Advantage (MA) / Part D Contract and Enrollment Data web page provides publicly available data on contracts and plans, enrollment numbers, service area data, and contact information for MA, Prescription Drug Plan (PDP), cost, PACE, and demonstration organizations. Information is available at the state, county, and contract level. These reports are updated monthly and many are available for the most current month back to mid-2006. Part D program data for 2006 and 2007, however, may not be available in the same time increments as later years.
2005 and Earlier
For data years 2005 and earlier, use the reports found on the CMS Health Plans, Reports, Files and Data web page.
The Kaiser Family Foundation's Medicare Health and Prescription Drug Plan Tracker has an easy to use online tool that allows you to compare Medicare enrollment, including penetration rate, at the state, county, MSA, and MA region. The tool includes information from 1999 through the current year.