Dave Dohrmann http://bulletin.accurateshooter.com/2007/03/superfeet-for-your-front-rest/ used to make them, but he passed away a few years ago. I'm not sure what happened to his products after that. I'd contact John Perkins at 21st Century Shooting and see if he'll make some. He's always looking to expand his line. Send him the site addressed for accurate shooter referenced above for a picture of what you're looking for. Who knows, he may be interested in making some.
Get some of the benchrite-stabilfeet from Sinclair. Then send me your mailing address and I'll mail you 3 retainers with the springs attached. I had a complete set at one time, ended up selling just the feet. Kept the retainers and springs just in case. If you want them you can have them for free. All you have to do is drill 2 small holes in the feet without going through the neoprene base and then attach the end of the spring.