Somewhere on the net. I saw oversized locking bolts available, possibly Brownells.
As said above, start with a detail strip and clean. This will likely increase your problem to some degree, but it will tell you what's wrong, and what parts you need to replace.
As a temporary fix, you can peen the sides of the locking bolt where it slides in the receiver, thus tightenning it up. This has to be done very carefully, or you'll be stoning them down again.