They make some pretty good orthopedic dog beds. They are high dollar though (probably in the $200-300 range for the size a lab needs).
With that said - if you're anywhere near Canton, First Monday is this weekend. There is a guy there that sells dog beds - on the main part where all the regular stuff is (not dog alley) that is by the creek between the pavilions and the large open area. From time to time he will have orthopedic beds at a much better price than you can find anywhere. We got one a few years back and it's still holding up well. It is also a good place to get a regular dog bed if anyone is looking - think the regular ones are like $25 - he over stuffs them and IMO they're better than anything in the stores.