You can order followers and other fatory parts directly from Ruger.
You may also want to check Brownells . Click on " Products", select "Ruger" from the manufacturer drop-down box and search for "magazine follower." You'll probably get a couple pages of results, just look them over and find your application.
While your replacing followers, you might want to go ahead and replace the springs as well. Brownells has the springs too. Just search for them like you do for the followers.
To take it a step further, perhaps you would be interested in extra power magazine springs. If so try Wolff .