This site offers terrific near-live pricing for mortgage backed securities. They can be found by visiting the home page and looking at the right hand side of the screen. Click "MBS Quotes", and scroll down to the bottom of the page. This information is extremely accurate, and used by many of us in the industry.

http://www.mortgagenewsdaily.com/mortgage_rates/

By scrolling through the MBS blog here at MND, you can also see historical analysis and trends.

Many of the "pay" financial sites will also offer graphing capacity for historical values, and the agencies (fannie mae and freddie mac) offer investor information and support on their respective websites.

Finance sites such as google, bloomberg, or yahoo will have information on specific pools and performance.