You might be able to find tempered spring steel rods in the proper diameter at a regular old hardware store. You'd also need something like a dremel or grinder to cut the rod and polish it off a bit.
You'd need to determine how deep the groove is, to decide on how much of the spring will be hidden, and then determine how long the exposed part of the spring needs to be. You might be able to tell from wear on the lockbar how long the exposed part of the spring might have been. Often there's a ridge on the lockbar behind the pivot tht helps keep the spring from falling out, and should also give a clue as to a proper length for the replacement spring.
The trickiest part would be to push the spring into position, which has to be done with the blade and lockbar in the open position, using perhaps a couple of flat metal tools, one to compress the spring, and one to push it into the groove as it's being compressed. This can be a very tricky part, maneuvering the spring into a place where it can be pushed into the slot. The knife should be immobilized for this procedure. Make sure the edge is well taped off to keep from accidentally cutting yourself should your hands/work slip.
Keep in mind that it's hard to tell why the original spring managed to disappear. There may be another issue that keeps the fix from being simple.
