I just received a couple of these: Universal Quick Release Plate Clamp Knob 1/4" & 3/8" Screw Hole for Arca Swiss | eBay They are A-S clamps with a locking lever feature at $13.50 each. They must be a fairly new product as I have been buying A-S hardware on eBay for some time and the only other locking lever clamp I have seen is around $50.
If you've been holding off on converting to A-S because of the cost, I think this clamp solves the problem. I also have an Acratech locking lever clamp (GP/GPS Quick Release Locking Lever Clamp) on my GP ball head. The Acratech clamp retails for $150 and the only functional or quality difference from the eBay one is that the Acratech includes a bubble level. Both are far superior to the RRS lever clamp, which has no lever lock and hence has high potential for dropping your camera in the field.
If you've been holding off on converting to A-S because of the cost, I think this clamp solves the problem. I also have an Acratech locking lever clamp (GP/GPS Quick Release Locking Lever Clamp) on my GP ball head. The Acratech clamp retails for $150 and the only functional or quality difference from the eBay one is that the Acratech includes a bubble level. Both are far superior to the RRS lever clamp, which has no lever lock and hence has high potential for dropping your camera in the field.
Actually, that can be a little problematical: Plates with squishy padding are not a good idea because that compliance reduces the rigidity of the whole assembly. The ideal plate would be integral to the camera frame. Plates that can twist are also a problem, because they will twist and loosen. Blue loctite and tightening with a screwdriver or a wrench can work but not 100% in my experience. So you need a bare metal plate with some kind of lip or stop that catches the front edge of the camera base. (The RRS B-9 plate looks from its picture like an example that meets these requirements. 
